Release version 2.1.0

name and title were translated at render time but never machine-filled,
on the grounds that no backend tells the menu label "Pictures" from the
brand "Mastodon". That left the visible half of a card in English. They
are filled now, and the two key sets collapse into one.

The brands need somewhere to be named instead. app/i18n/keep.txt lists
them, one per line, and every entry is stored as itself in every target
language: no request, and never over an entry written by hand. --keep adds
one-off strings, --keep-file points elsewhere.

The shipped list holds the 43 product names that appear as name: or title:
in config.sample.yaml. Generic labels — Pictures, Imprint, Settings,
Certificates — are deliberately absent, and so are Cybermaster, Polymath
and Yachtmaster, which read as brand or as job title depending on who is
asking.

Two of these behaviours first shipped unguarded. A test that protected a
string and asserted the hand-written value survived passed either way,
because a run where nothing is missing reports "complete" and never
writes; and nothing exercised main(), so the keep file could stop being
read without a failure. Both are covered.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/lint.yml

@@ -19,6 +19,61 @@ jobs:
       - name: Run actionlint
         run: docker run --rm -v "$PWD:/repo" -w /repo rhysd/actionlint:latest
 
+  lint-yaml:
+    name: Lint YAML
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.12"
+
+      - name: Install lint dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install ".[dev]"
+
+      - name: Run yamllint
+        run: yamllint --strict .
+
+  lint-js:
+    name: Lint JavaScript
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: npm
+          cache-dependency-path: app/package.json
+
+      - name: Install Node dependencies
+        working-directory: app
+        run: npm install
+
+      - name: Run eslint
+        working-directory: app
+        run: npx eslint .
+
+  lint-shell:
+    name: Lint shell scripts
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+
+      - name: Run shellcheck
+        run: docker run --rm -v "$PWD:/mnt" -w /mnt koalaman/shellcheck:stable scripts/*.sh
+
   lint-python:
     name: Lint Python
     runs-on: ubuntu-latest

.gitignore

@@ -1,4 +1,5 @@
 app/config.yaml
+app/i18n/content/
 *__pycache__*
 app/static/cache/*
 .env
@@ -10,3 +11,4 @@ hadolint-results.sarif
 build/
 *.egg-info/
 app/core.*
+.mcp.json

.yamllint

@@ -0,0 +1,23 @@
+---
+# Correctness only. No `extends: default`, so nothing but the rules below runs:
+# the repository predates this linter and its cosmetic findings (indentation,
+# line length, trailing spaces) would be noise nobody acts on.
+#
+# key-duplicates is the rule that earns its keep: PyYAML keeps the last of two
+# identical keys without complaining, so a duplicated entry in a translation
+# catalogue silently drops a translation.
+ignore: |
+  .git/
+  .venv/
+  venv/
+  node_modules/
+  app/node_modules/
+  app/static/vendor/
+  build/
+  .ruff_cache/
+
+rules:
+  key-duplicates: enable
+  octal-values:
+    forbid-implicit-octal: true
+    forbid-explicit-octal: true

CHANGELOG.md

@@ -1,3 +1,16 @@
+# Changelog
+
+## [2.1.0] - 2026-08-22
+
+* Multilingual site: every ISO 639-1 language has its own URL, */* follows the visitor's browser language, and a switcher in the navbar lists all 184 in their own script
+* Right-to-left layouts: Arabic, Hebrew, Persian, Urdu and six more mirror the page automatically
+* Machine translation: *make i18n* fills your card and menu texts from a LibreTranslate instance, *make i18n-ui* the interface strings; hand-corrected entries are never overwritten
+* Brand protection: list your product names in *app/i18n/keep.txt* and they stay untranslated in every language
+* Search engines: each page declares a canonical URL and an *hreflang* alternate per language
+* Security: configuration and translations are HTML-escaped, script URLs are stripped from links, images and the iframe view — set *TRUSTED_HOSTS* in *.env* to pin your public hostname
+* Reliability: a slow NASA APOD lookup no longer takes the page down
+* Developer tooling: *make test-e2e* runs Cypress directly instead of through act, and YAML, JavaScript and shell now have linters in CI
+
 ## [2.0.0] - 2026-05-18
 
 * * Asset resolution: new probe-first resolver tries a HEAD request and embeds reachable image URLs directly via a new external_url field, falling back to the cache-download path only when the probe fails; broken /static/https://... URLs no longer appear when the source cannot be downloaded

Makefile

@@ -78,6 +78,8 @@ run-dev: env config config
 			-p "$$PORT:$$PORT" \
 			--name portfolio \
 			-v "$(PWD)/app/:/app" \
+			-e PORT="$$PORT" \
+			-e TRUSTED_HOSTS="$$TRUSTED_HOSTS" \
 			-e FLASK_APP=app.py \
 			-e FLASK_ENV=development \
 			"$$IMAGE_NAME"
@@ -89,6 +91,8 @@ run-prod: env config config
 		docker run -d \
 			-p "$$PORT:$$PORT" \
 			--name portfolio \
+			-e PORT="$$PORT" \
+			-e TRUSTED_HOSTS="$$TRUSTED_HOSTS" \
 			"$$IMAGE_NAME"
 
 .PHONY: logs
@@ -132,6 +136,23 @@ install-dev:
 	# Install runtime and developer dependencies from pyproject.toml.
 	$(PYTHON) -m pip install -e ".[dev]"
 
+.PHONY: i18n
+i18n: env config
+	# Fill missing content translations in app/i18n/content/ via LibreTranslate.
+	@$(call _require_env,LIBRETRANSLATE_URL); \
+		$(PYTHON) utils/i18n_sync.py \
+			--url "$$LIBRETRANSLATE_URL" \
+			--api-key "$$LIBRETRANSLATE_API_KEY"
+
+.PHONY: i18n-ui
+i18n-ui: env
+	# Fill missing interface translations in app/i18n/ui/ via LibreTranslate.
+	@$(call _require_env,LIBRETRANSLATE_URL); \
+		$(PYTHON) utils/i18n_sync.py \
+			--catalog ui \
+			--url "$$LIBRETRANSLATE_URL" \
+			--api-key "$$LIBRETRANSLATE_API_KEY"
+
 .PHONY: lint-actions
 lint-actions:
 	# Lint GitHub Actions workflows.
@@ -148,6 +169,21 @@ lint-docker:
 	# Lint the Dockerfile.
 	docker run --rm -i hadolint/hadolint < Dockerfile
 
+.PHONY: lint-yaml
+lint-yaml: install-dev
+	# Lint YAML for duplicate keys and ambiguous scalars (see .yamllint).
+	$(PYTHON) -m yamllint --strict .
+
+.PHONY: lint-js
+lint-js: node-deps
+	# Lint the browser and Cypress JavaScript.
+	cd app && env -u ELECTRON_RUN_AS_NODE npx eslint .
+
+.PHONY: lint-shell
+lint-shell:
+	# Lint the shell scripts.
+	docker run --rm -v "$$PWD:/mnt" -w /mnt koalaman/shellcheck:stable scripts/*.sh
+
 .PHONY: test-lint
 test-lint:
 	# Run lint guardrail tests.
@@ -169,7 +205,7 @@ test-security: install
 	$(PYTHON) -m unittest discover -s tests/security -t .
 
 .PHONY: lint
-lint: lint-actions lint-python lint-docker test-lint
+lint: lint-actions lint-python lint-yaml lint-js lint-docker lint-shell test-lint
 	# Run the full lint suite.
 
 .PHONY: security
@@ -179,9 +215,20 @@ security: install-dev test-security
 	$(PYTHON) utils/export_runtime_requirements.py > /tmp/portfolio-runtime-requirements.txt
 	$(PYTHON) -m pip_audit -r /tmp/portfolio-runtime-requirements.txt
 
+.PHONY: node-deps
+node-deps:
+	# Install the Cypress binary and the browser vendor assets into app/.
+	cd app && npm install
+
 .PHONY: test-e2e
-test-e2e:
-	# Run Cypress end-to-end tests via act (stop portfolio container to free port first).
+test-e2e: env config node-deps
+	# Run Cypress against a locally started Flask app — no act, no runner image.
+	@$(call _require_env,PORT); \
+		PORT="$$PORT" PYTHON="$(PYTHON)" scripts/run-e2e.sh
+
+.PHONY: test-e2e-act
+test-e2e-act:
+	# Run the CI end-to-end job through act (stop portfolio container to free port first).
 	-docker stop portfolio 2>/dev/null || true
 	$(ACT) workflow_dispatch -W .github/workflows/tests.yml -j e2e
 	-docker start portfolio 2>/dev/null || true

README.md

@@ -22,6 +22,8 @@ A lightweight, Docker-powered portfolio/landing-page generator—fully customiza
   Auto-cache assets for lightning-fast loading.  
 - **Responsive Design**  
   Built on Bootstrap; looks great on desktop, tablet & mobile.  
+- **184 Languages**  
+  Every ISO 639-1 code, browser-negotiated, RTL-aware, with machine translation for your own content.  
 - **YAML-Driven**  
   All content & structure defined in a simple `config.yaml`.  
 - **CLI Control**  
@@ -139,12 +141,75 @@ company:
 
 ---
 
+## 🌍 Languages
+
+Every ISO 639-1 language — all 184 two-letter codes — has a URL, a display
+name in its own script and a writing direction. The interface ships translated
+for 29 of them; the rest fall back to English string by string until a
+catalogue is filled. `/` serves the best match for the visitor's
+`Accept-Language` header, `/<code>/` forces one, and a switcher in the navbar
+lists them all. The ten right-to-left languages get `dir="rtl"` and Bootstrap's RTL
+stylesheet automatically.
+
+Translations live in two catalogues, both keyed by the English source string:
+
+| Path | Tracked | Holds |
+| --- | --- | --- |
+| `app/i18n/ui/<code>.yaml` | yes | Interface strings. Shipped for 29 languages; English is the source and has no file. |
+| `app/i18n/content/<code>.yaml` | no | Your `config.yaml` prose, generated per deployment. |
+
+A string with no catalogue entry falls back to English, so a half-filled
+catalogue degrades instead of breaking.
+
+Fill the content catalogues from a [LibreTranslate](https://libretranslate.com/)
+instance — set `LIBRETRANSLATE_URL` in `.env`, then:
+
+```bash
+make i18n
+```
+
+This fills the interface strings of the languages that ship no catalogue as
+well. Existing entries are never overwritten, and a string the shipped
+catalogue already covers is never requested, so corrections you make by hand
+survive later runs.
+
+`name`, `title`, `description`, `text`, `warning`, `info` and `subtitel` are
+translated; `url`, `link_text`, `identifier` and icon classes never are.
+
+A machine cannot tell the menu label "Pictures" from the brand "Mastodon", so
+list the brands in `app/i18n/keep.txt`, one per line — they are then stored as
+themselves in every language and cost no request:
+
+```
+# Strings utils/i18n_sync.py stores as themselves instead of translating.
+Mastodon
+Nextcloud
+freelancermap.de
+```
+
+Add one-off entries with `--keep Foo Bar`, or point somewhere else with
+`--keep-file`. A protected string never replaces an entry you already wrote by
+hand.
+
+---
+
 ## 🚢 Production Deployment
 
 * Use a reverse proxy (NGINX/Apache).
 * Secure with SSL/TLS.
 * Swap to a production database if needed.
 
+Because every page carries a canonical URL and 184 `hreflang` alternates, two
+details of the proxy setup now matter:
+
+* **Set `TRUSTED_HOSTS`** in `.env` to your public hostname(s), comma-separated.
+  Left empty, the app reflects whatever `Host` header arrives into its canonical,
+  `hreflang` and redirect URLs — so a shared cache in front of it can be made to
+  store a redirect pointing somewhere else.
+* **Have the proxy send `X-Forwarded-Proto`.** Without it the app cannot know TLS
+  terminated upstream and every canonical URL claims `http://`. `X-Forwarded-Host`
+  is deliberately *not* trusted; set `Host` to the public name instead.
+
 ---
 
 ## 📜 License

app/app.py

@@ -3,10 +3,12 @@ import os
 
 import requests
 import yaml
-from flask import Flask, current_app, render_template, url_for
+from flask import Flask, current_app, make_response, render_template, request, url_for
 from markupsafe import Markup
+from werkzeug.middleware.proxy_fix import ProxyFix
 
 try:
+    from app.utils import i18n
     from app.utils.asset_resolver import asset_src, resolve_asset_cache
     from app.utils.cache_manager import CacheManager
     from app.utils.compute_card_classes import compute_card_classes
@@ -17,6 +19,10 @@ except ImportError:  # pragma: no cover - supports running from the app/ directo
     from utils.compute_card_classes import compute_card_classes
     from utils.configuration_resolver import ConfigurationResolver
 
+    from utils import i18n
+
+TRANSLATED_SECTIONS = ("cards", "company", "navigation", "platform")
+
 logging.basicConfig(level=logging.DEBUG)
 
 FLASK_ENV = os.getenv("FLASK_ENV", "production")
@@ -42,6 +48,8 @@ def load_config(app):
     resolver = ConfigurationResolver(config)
     resolver.resolve_links()
     app.config.update(resolver.get_config())
+    app.config["TRANSLATED_CONFIG"] = {}
+    i18n.clear_catalogs()
 
 
 def cache_icons_and_logos(app):
@@ -60,6 +68,23 @@ def cache_icons_and_logos(app):
 # Initialize Flask app
 app = Flask(__name__)
 
+app.jinja_options = {**app.jinja_options, "autoescape": True}
+
+app.wsgi_app = ProxyFix(app.wsgi_app, x_for=0, x_proto=1)
+
+
+def trusted_hosts(raw):
+    """Parse a comma-separated host list, or None when nothing is configured.
+
+    Args:
+        raw: the ``TRUSTED_HOSTS`` value, possibly empty.
+    """
+    hosts = [host.strip() for host in raw.split(",") if host.strip()]
+    return hosts or None
+
+
+app.config["TRUSTED_HOSTS"] = trusted_hosts(os.getenv("TRUSTED_HOSTS", ""))
+
 # Load configuration and cache assets on startup
 load_config(app)
 cache_icons_and_logos(app)
@@ -91,36 +116,79 @@ def reload_config_in_dev():
         cache_icons_and_logos(app)
 
 
-@app.route("/")
-def index():
-    """Render the main index page."""
-    cards = app.config["cards"]
-    lg_classes, md_classes = compute_card_classes(cards)
-    apod_bg = None
+def translated_config(lang):
+    """Return the configuration sections translated into ``lang``, memoized.
+
+    The memo is dropped by ``load_config``, so a development reload picks up
+    edited content on the next request.
+    """
+    memo = app.config["TRANSLATED_CONFIG"]
+    if lang not in memo:
+        source = {section: app.config[section] for section in TRANSLATED_SECTIONS}
+        memo[lang] = i18n.translate_tree(source, lang)
+    return memo[lang]
+
+
+def apod_background():
+    """Return today's NASA APOD image URL, or None when unavailable."""
     api_key = app.config.get("NASA_API_KEY")
-    if api_key:
+    if not api_key:
+        return None
+
+    try:
         resp = requests.get(
             "https://api.nasa.gov/planetary/apod",
             params={"api_key": api_key},
             timeout=10,
         )
-        if resp.ok:
+    except requests.RequestException:
+        logging.warning("APOD lookup failed", exc_info=True)
+        return None
+
+    if not resp.ok:
+        return None
+
     data = resp.json()
-            if data.get("media_type") == "image":
-                apod_bg = data.get("url")
+    return data.get("url") if data.get("media_type") == "image" else None
+
+
+def render_index(lang):
+    """Render the index page in ``lang``."""
+    config = translated_config(lang)
+    cards = config["cards"]
+    lg_classes, md_classes = compute_card_classes(cards)
 
     return render_template(
         "pages/index.html.j2",
         cards=cards,
-        company=app.config["company"],
-        navigation=app.config["navigation"],
-        platform=app.config["platform"],
+        company=config["company"],
+        navigation=config["navigation"],
+        platform=config["platform"],
         lg_classes=lg_classes,
         md_classes=md_classes,
-        apod_bg=apod_bg,
+        apod_bg=apod_background(),
+        lang=lang,
+        lang_dir=i18n.direction(lang),
+        languages=i18n.LANGUAGES,
+        ui_strings=i18n.ui_strings(lang),
+        t=lambda source: i18n.catalog(lang).get(source, source),
     )
 
 
+@app.route("/")
+def index():
+    """Render the index page in the language the browser asks for."""
+    response = make_response(render_index(i18n.negotiate(request.accept_languages)))
+    response.headers["Vary"] = "Accept-Language"
+    return response
+
+
+@app.route(f"/<any({','.join(i18n.LANGUAGES)}):lang>/")
+def localized_index(lang):
+    """Render the index page in an explicitly requested language."""
+    return render_index(lang)
+
+
 if __name__ == "__main__":
     app.run(
         debug=(FLASK_ENV == "development"),

app/cypress/e2e/i18n.spec.js

@@ -0,0 +1,243 @@
+// cypress/e2e/i18n.spec.js
+
+const GERMAN_BROWSER = { headers: { 'Accept-Language': 'de-DE,de;q=0.9,en;q=0.8' } };
+
+describe('Language negotiation', () => {
+  it('serves English to an English browser', () => {
+    cy.visit('/', { headers: { 'Accept-Language': 'en-US,en;q=0.9' } });
+    cy.get('html').should('have.attr', 'lang', 'en');
+    cy.get('footer.footer a.iframe-link').should('contain.text', 'Imprint');
+  });
+
+  it('serves German to a German browser', () => {
+    cy.visit('/', GERMAN_BROWSER);
+    cy.get('html').should('have.attr', 'lang', 'de');
+    cy.get('footer.footer a.iframe-link').should('contain.text', 'Impressum');
+  });
+
+  it('falls back to English for an unsupported browser language', () => {
+    cy.visit('/', { headers: { 'Accept-Language': 'xx-XX' } });
+    cy.get('html').should('have.attr', 'lang', 'en');
+  });
+
+  it('lets the URL prefix override the browser language', () => {
+    cy.visit('/fr/', GERMAN_BROWSER);
+    cy.get('html').should('have.attr', 'lang', 'fr');
+    cy.get('footer.footer a.iframe-link').should('contain.text', 'Mentions légales');
+  });
+
+  it('rejects an unsupported language code', () => {
+    cy.request({ url: '/xx/', failOnStatusCode: false })
+      .its('status')
+      .should('eq', 404);
+  });
+
+  it('does not turn unrelated single-segment paths into redirects', () => {
+    ['/robots.txt', '/favicon.ico', '/sitemap.xml'].forEach(path => {
+      cy.request({ url: path, failOnStatusCode: false, followRedirect: false })
+        .its('status')
+        .should('eq', 404);
+    });
+  });
+
+  it('marks the negotiated route as varying by language', () => {
+    cy.request('/')
+      .its('headers.vary')
+      .should('contain', 'Accept-Language');
+  });
+});
+
+describe('Language switcher', () => {
+  beforeEach(() => {
+    cy.viewport(1280, 720);
+    cy.visit('/en/');
+  });
+
+  it('names the active language and offers every ISO 639-1 code', () => {
+    cy.get('#navbarDropdownLanguage')
+      .should('have.attr', 'data-bs-toggle', 'dropdown')
+      .and('contain.text', 'English');
+
+    cy.get('#navbarDropdownLanguage')
+      .parent('.nav-item')
+      .find('> .dropdown-menu a.dropdown-item')
+      .should('have.length', 184);
+  });
+
+  it('marks the active language', () => {
+    cy.get('#navbarDropdownLanguage').click();
+    cy.get('.dropdown-menu a.dropdown-item.active[hreflang="en"]').should('exist');
+  });
+
+  it('navigates to the chosen language', () => {
+    cy.get('#navbarDropdownLanguage').click();
+    cy.get('.dropdown-menu a.dropdown-item[hreflang="de"]')
+      .should('have.text', 'Deutsch')
+      .click();
+
+    cy.url().should('match', /\/de\/$/);
+    cy.get('html').should('have.attr', 'lang', 'de');
+  });
+});
+
+describe('Translated interface strings', () => {
+  it('translates the strings rendered by the templates', () => {
+    cy.visit('/de/');
+    cy.get('#dynamicCopyButton').should('have.text', 'Kopieren');
+    cy.get('#dynamicChildrenSection h6').should('have.text', 'Optionen:');
+    cy.get('#dynamicAlternativesSection h6').should('have.text', 'Alternativen:');
+    cy.get('.modal-footer button').should('have.text', 'Schließen');
+  });
+
+  it('exposes the catalogue to client-side code', () => {
+    cy.visit('/de/');
+    cy.window().its('I18N').should('deep.include', {
+      Open: 'Öffnen',
+      'Open Link': 'Link öffnen',
+      'Identifier copied to clipboard!': 'Kennung in die Zwischenablage kopiert!',
+    });
+  });
+
+  it('leaves the catalogue untranslated in the source language', () => {
+    cy.visit('/en/');
+    cy.window().its('I18N').should('deep.include', { Open: 'Open' });
+  });
+});
+
+describe('Strings translated by modal.js', () => {
+  const item = {
+    name: 'Test Item',
+    identifier: 'ABC123',
+    icon: { class: 'fa fa-test' },
+    alternatives: [
+      { name: 'Alt One', identifier: 'ALT1', icon: { class: 'fa fa-alt1' } },
+    ],
+  };
+
+  beforeEach(() => {
+    cy.visit('/de/');
+    cy.window().then(win => {
+      cy.stub(win.navigator.clipboard, 'writeText').resolves();
+      cy.stub(win, 'alert');
+    });
+  });
+
+  it('translates the button of a list entry', () => {
+    cy.window().invoke('openDynamicPopup', item);
+    cy.get('#dynamicAlternativesList button').should('have.text', 'Öffnen');
+  });
+
+  it('translates the link label when the entry has no description', () => {
+    cy.window().invoke('openDynamicPopup', {
+      ...item,
+      url: 'https://example.com',
+      description: null,
+    });
+    cy.get('#dynamicModalLinkHref').should('have.text', 'Link öffnen');
+  });
+
+  it('translates the clipboard confirmation', () => {
+    cy.window().invoke('openDynamicPopup', item);
+    cy.get('#dynamicCopyButton').click();
+    cy.window()
+      .its('alert')
+      .should('have.been.calledWith', 'Kennung in die Zwischenablage kopiert!');
+  });
+});
+
+describe('Right-to-left languages', () => {
+  it('flips the document and loads the RTL stylesheet', () => {
+    cy.visit('/ar/');
+    cy.get('html').should('have.attr', 'dir', 'rtl');
+    cy.get('link[href*="bootstrap.rtl.min.css"]').should('exist');
+    cy.get('link[href*="vendor/bootstrap/css/bootstrap.min.css"]').should('not.exist');
+    cy.get('body').should('have.css', 'direction', 'rtl');
+  });
+
+  it('keeps left-to-right languages on the default stylesheet', () => {
+    cy.visit('/en/');
+    cy.get('html').should('have.attr', 'dir', 'ltr');
+    cy.get('link[href*="bootstrap.rtl.min.css"]').should('not.exist');
+  });
+
+  it('actually serves the RTL stylesheet it links to', () => {
+    cy.visit('/ar/');
+    cy.get('link[href*="bootstrap.rtl.min.css"]')
+      .should('have.attr', 'href')
+      .then(href => {
+        cy.request(href).its('status').should('eq', 200);
+      });
+  });
+});
+
+describe('Translated interface details', () => {
+  it('translates the strings only a screen reader sees', () => {
+    cy.visit('/de/');
+
+    cy.get('#dynamicModal .btn-close').should('have.attr', 'aria-label', 'Schließen');
+  });
+
+  it('translates the alert headings', () => {
+    cy.visit('/fr/');
+
+    cy.get('#dynamicModalWarning h5').should('contain.text', 'Avertissement');
+    cy.get('#dynamicModalInfo h5').should('contain.text', 'Informations');
+  });
+
+  it('translates the language switcher tooltip', () => {
+    cy.visit('/de/');
+
+    cy.get('#navbarDropdownLanguage').should('have.attr', 'title', 'Sprache');
+  });
+
+  it('tags each switcher entry with its own language', () => {
+    cy.visit('/en/');
+
+    cy.get('.dropdown-menu a.dropdown-item[hreflang="ja"]').should(
+      'have.attr',
+      'lang',
+      'ja',
+    );
+  });
+
+  it('scrolls inside the language menu instead of past the page', () => {
+    cy.viewport(1280, 720);
+    cy.visit('/en/');
+    cy.get('#navbarDropdownLanguage').click();
+
+    cy.get('.dropdown-menu.language-menu').should($menu => {
+      const menu = $menu[0];
+      expect(menu.scrollHeight, 'taller than it shows').to.be.greaterThan(
+        menu.clientHeight,
+      );
+      expect(menu.getBoundingClientRect().height).to.be.lessThan(720);
+      expect(getComputedStyle(menu).overflowY).to.eq('auto');
+    });
+  });
+
+  it('offers the switcher in the header only', () => {
+    cy.viewport(1280, 720);
+    cy.visit('/en/');
+
+    cy.get('#navbarNavheader #navbarDropdownLanguage').should('exist');
+    cy.get('#navbarNavfooter #navbarDropdownLanguage').should('not.exist');
+  });
+});
+
+describe('Search engine metadata', () => {
+  beforeEach(() => {
+    cy.visit('/de/');
+  });
+
+  it('declares an alternate for every language plus a default', () => {
+    cy.get('link[rel="alternate"][hreflang]').should('have.length', 185);
+    cy.get('link[rel="alternate"][hreflang="x-default"]').should('exist');
+    cy.get('link[rel="alternate"][hreflang="ja"]')
+      .should('have.attr', 'href')
+      .and('match', /\/ja\/$/);
+  });
+
+  it('points the canonical URL at the language actually served', () => {
+    cy.get('link[rel="canonical"]').should('have.attr', 'href').and('match', /\/de\/$/);
+  });
+});

app/cypress/e2e/injection.spec.js

@@ -0,0 +1,263 @@
+// cypress/e2e/injection.spec.js
+
+describe('Untrusted content in the modal', () => {
+  const base = {
+    name: 'Test Item',
+    identifier: 'ABC123',
+    icon: { class: 'fa fa-test' },
+  };
+
+  beforeEach(() => {
+    cy.visit('/');
+    cy.window().then(win => {
+      cy.stub(win.navigator.clipboard, 'writeText').resolves();
+      cy.stub(win, 'alert');
+    });
+  });
+
+  function open(item = {}) {
+    cy.window().invoke('openDynamicPopup', { ...base, ...item });
+  }
+
+  describe('markdown rendered into innerHTML', () => {
+    it('strips a plain script URL', () => {
+      open({
+        warning: '[click me](javascript:window.__xss = true)',
+        info: '![x](data:text/html;base64,PHNjcmlwdD4=)',
+      });
+
+      cy.get('#dynamicModalWarningText').find('a').should('not.exist');
+      cy.get('#dynamicModalWarningText').should('contain.text', 'click me');
+      cy.get('#dynamicModalInfoText').find('img').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('strips a script URL hidden behind character references', () => {
+      open({
+        warning:
+          '[a](javascript:window.__xss=1) [b](javascript:window.__xss=1)',
+        info: '[c](java	script:window.__xss=1) [d](java
script:window.__xss=1)',
+      });
+
+      cy.get('#dynamicModalWarningText').find('a').should('not.exist');
+      cy.get('#dynamicModalInfoText').find('a').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('strips a script URL written as a reference-style link', () => {
+      open({
+        warning: '[click me][ref]\n\n[ref]: javascript:window.__xss=1',
+      });
+
+      cy.get('#dynamicModalWarningText').find('a').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('neutralises raw HTML', () => {
+      open({
+        warning: '<img src=x onerror="window.__xss = true">',
+        info: '<a href="javascript:window.__xss = true">x</a>',
+      });
+
+      cy.get('#dynamicModalWarningText').find('img').should('not.exist');
+      cy.get('#dynamicModalWarningText').should('contain.text', 'onerror');
+      cy.get('#dynamicModalInfoText').find('a').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('neutralises raw HTML used as the text of a stripped link', () => {
+      open({ warning: '[<img src=x onerror="window.__xss = true">](javascript:bad)' });
+
+      cy.get('#dynamicModalWarningText').find('img').should('not.exist');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('keeps a relative link', () => {
+      open({ warning: 'See [the notes](/#anchor)' });
+
+      cy.get('#dynamicModalWarningText')
+        .find('a')
+        .should('have.attr', 'href', '/#anchor');
+    });
+
+    it('keeps the text of a link it strips', () => {
+      open({ warning: '[read this](javascript:window.__xss=1)' });
+
+      cy.get('#dynamicModalWarningText').should('contain.text', 'read this');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('keeps ordinary markdown', () => {
+      open({ warning: 'See [Matrix](https://matrix.org/) and **mind** this' });
+
+      cy.get('#dynamicModalWarningText')
+        .find('a')
+        .should('have.attr', 'href', 'https://matrix.org/');
+      cy.get('#dynamicModalWarningText').find('strong').should('have.text', 'mind');
+    });
+  });
+
+  describe('values interpolated outside markdown', () => {
+    it('does not treat an interface string as markup', () => {
+      cy.window().then(win => {
+        win.I18N.Open = '<img src=x onerror="window.__xss = true">';
+      });
+      open({
+        alternatives: [
+          { name: 'Alt', identifier: 'A', icon: { class: 'fa-alt' } },
+        ],
+      });
+
+      cy.get('#dynamicAlternativesList').find('img').should('not.exist');
+      cy.get('#dynamicAlternativesList').should('contain.text', 'onerror');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('falls back to the English source when a string is missing', () => {
+      cy.window().then(win => {
+        delete win.I18N;
+      });
+      open({
+        alternatives: [
+          { name: 'Alt', identifier: 'A', icon: { class: 'fa-alt' } },
+        ],
+      });
+
+      cy.get('#dynamicAlternativesList button').should('have.text', 'Open');
+    });
+
+    it('renders no placeholder for a missing name', () => {
+      open({ name: undefined });
+
+      cy.get('#dynamicModalLabel').should('not.contain.text', 'undefined');
+    });
+
+    it('does not treat the name or the icon class as markup', () => {
+      open({
+        name: '<img src=x onerror="window.__xss = true">',
+        icon: { class: 'fa" onmouseover="window.__xss = true' },
+        alternatives: [
+          {
+            name: '<img src=y onerror="window.__xss = true">',
+            identifier: 'ALT1',
+            icon: { class: 'fa-alt' },
+          },
+        ],
+      });
+
+      cy.get('#dynamicModalLabel').find('img').should('not.exist');
+      cy.get('#dynamicModalLabel').should('contain.text', 'onerror');
+      cy.get('#dynamicAlternativesList').find('img').should('not.exist');
+      cy.get('#dynamicAlternativesList').should('contain.text', 'onerror');
+      cy.window().should('not.have.property', '__xss');
+    });
+  });
+
+  describe('the link the modal offers', () => {
+    it('drops a URL that uses an unsafe scheme', () => {
+      open({ url: 'javascript:window.__xss = true', description: 'Bad' });
+
+      cy.get('#dynamicModalLinkHref').should('not.have.attr', 'href');
+      cy.get('#dynamicModalLinkHref').should('have.text', 'Bad');
+      cy.window().should('not.have.property', '__xss');
+    });
+
+    it('keeps an ordinary URL', () => {
+      open({ url: 'https://example.com', description: 'Good' });
+
+      cy.get('#dynamicModalLinkHref').should(
+        'have.attr',
+        'href',
+        'https://example.com',
+      );
+    });
+
+    it('keeps a URL that carries surrounding whitespace', () => {
+      open({ url: ' https://example.com ', description: 'Good' });
+
+      cy.get('#dynamicModalLinkHref').should('have.attr', 'href');
+    });
+
+    it('keeps a mailto URL', () => {
+      open({ url: 'mailto:kevin@veen.world', description: 'Write' });
+
+      cy.get('#dynamicModalLinkHref').should(
+        'have.attr',
+        'href',
+        'mailto:kevin@veen.world',
+      );
+    });
+
+    it('restores the link after a popup whose URL was dropped', () => {
+      open({ url: 'javascript:window.__xss = true', description: 'Bad' });
+      cy.get('#dynamicModalLinkHref').should('not.have.attr', 'href');
+
+      open({ url: 'https://example.com', description: 'Good' });
+      cy.get('#dynamicModalLinkHref').should(
+        'have.attr',
+        'href',
+        'https://example.com',
+      );
+    });
+
+    it('does not let one popup iframe handler outlive it', () => {
+      open({ url: 'https://a.test/', description: 'A', iframe: true });
+      cy.get('#dynamicModalLinkHref').should('have.class', 'iframe');
+
+      open({ url: 'https://b.test/', description: 'B' });
+
+      cy.get('#dynamicModalLinkHref').should('not.have.class', 'iframe');
+      cy.get('#dynamicModalLinkHref').should($anchor => {
+        expect($anchor[0].onclick, 'stale click handler').to.equal(null);
+      });
+    });
+
+    it('opens the current popup URL, not an earlier one', () => {
+      open({ url: 'https://a.test/', description: 'A', iframe: true });
+      open({ url: 'https://b.test/', description: 'B', iframe: true });
+
+      cy.get('#dynamicModalLinkHref').click();
+
+      cy.get('#main')
+        .find('iframe', { timeout: 4000 })
+        .should('have.attr', 'src', 'https://b.test/');
+    });
+  });
+});
+
+describe('Untrusted content reaching the iframe', () => {
+  const AFTER_THE_FADE = 3000;
+
+  it('refuses to open a script URL handed over by the modal', () => {
+    cy.visit('/');
+    cy.window().invoke('openDynamicPopup', {
+      name: 'Bad',
+      icon: { class: 'fa fa-test' },
+      url: 'javascript:window.__xss = true',
+      description: 'Watch',
+      iframe: true,
+    });
+
+    cy.get('#dynamicModalLinkHref').click({ force: true });
+
+    cy.wait(AFTER_THE_FADE);
+    cy.get('#main').find('iframe').should('not.exist');
+    cy.window().should('not.have.property', '__xss');
+  });
+
+  it('refuses a script URL supplied through the query string', () => {
+    cy.visit('/?iframe=javascript:window.__xss%20%3D%20true');
+
+    cy.wait(AFTER_THE_FADE);
+    cy.get('#main').find('iframe').should('not.exist');
+    cy.window().should('not.have.property', '__xss');
+  });
+
+  it('still opens an ordinary URL from the query string', () => {
+    cy.visit('/?iframe=https://example.com/');
+
+    cy.get('#main')
+      .find('iframe', { timeout: AFTER_THE_FADE })
+      .should('have.attr', 'src', 'https://example.com/');
+  });
+});

app/eslint.config.js

@@ -0,0 +1,78 @@
+'use strict';
+/**
+ * Correctness only — eslint's recommended set, no stylistic rules.
+ *
+ * The browser scripts are plain <script> tags sharing one global scope: each
+ * file declares some functions and calls others declared elsewhere. That is why
+ * they are listed as globals and why no-redeclare is off — the declaring file
+ * would otherwise be reported for defining its own function.
+ */
+const js = require('@eslint/js');
+const globals = require('globals');
+
+// Vendored libraries plus the functions static/js files call across each other.
+// Keep this to names that really cross a file boundary. Every superfluous
+// entry is a permanent no-undef blind spot for a typo of that name.
+const SHARED = {
+  bootstrap: 'readonly',
+  marked: 'readonly',
+  $: 'readonly',
+  jQuery: 'readonly',
+  openDynamicPopup: 'readonly',
+  closeAllModals: 'readonly',
+  isSafeUrl: 'readonly',
+  openIframe: 'readonly',
+  enterFullscreen: 'readonly',
+  exitFullscreen: 'readonly',
+  setFullWidth: 'readonly',
+  initFullWidthFromUrl: 'readonly',
+  adjustScrollContainerHeight: 'readonly',
+  updateCustomScrollbar: 'readonly',
+};
+
+module.exports = [
+  { ignores: ['node_modules/**', 'static/vendor/**', 'cypress/screenshots/**'] },
+  {
+    files: ['static/js/**/*.js'],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: 'script',
+      globals: { ...globals.browser, ...SHARED },
+    },
+    rules: {
+      ...js.configs.recommended.rules,
+      'no-redeclare': 'off',
+      // vars: 'local' — a top-level function here is the API other files and
+      // the templates call, so only unused locals are a defect.
+      'no-unused-vars': [
+        'error',
+        { vars: 'local', args: 'none', caughtErrors: 'none' },
+      ],
+    },
+  },
+  {
+    files: ['cypress/**/*.js'],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: 'script',
+      globals: {
+        ...globals.browser,
+        ...globals.mocha,
+        cy: 'readonly',
+        Cypress: 'readonly',
+        expect: 'readonly',
+        assert: 'readonly',
+      },
+    },
+    rules: js.configs.recommended.rules,
+  },
+  {
+    files: ['scripts/**/*.js', 'cypress.config.js', 'eslint.config.js'],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: 'commonjs',
+      globals: globals.node,
+    },
+    rules: js.configs.recommended.rules,
+  },
+];

app/i18n/keep.txt

@@ -0,0 +1,46 @@
+# Strings utils/i18n_sync.py stores as themselves instead of translating.
+# One per line; blank lines and lines starting with # are ignored.
+
+Baserow
+Big Blue Button
+Bluesky
+Buy me a Coffee
+Discourse
+Duolingo
+Eversports
+Facebook
+Friendica
+Garmin
+Gitea
+GitHub
+GitHub Sponsors
+Infinito.Nexus
+Instagram
+Keycloak
+LDAP
+LinkedIn
+Mailu
+Mastodon
+Matomo
+Matrix
+Nextcloud
+Open Project
+Patreon
+PayPal
+Peertube
+Pixelfed
+Signal
+Snipe IT
+Spotify
+Taiga
+Telegram
+Twitter
+WhatsApp
+XING
+YouTube
+Yourls
+Zoom
+freelancermap.de
+malt
+phpMyAdmin
+upwork.com

app/i18n/ui/ar.yaml

@@ -0,0 +1,11 @@
+Alternatives: بدائل
+Close: إغلاق
+Copy: نسخ
+Identifier copied to clipboard!: تم نسخ المعرّف إلى الحافظة!
+Imprint: معلومات قانونية
+Information: معلومات
+Language: اللغة
+Open: فتح
+Open Link: فتح الرابط
+Options: خيارات
+Warning: تحذير

app/i18n/ui/bn.yaml

@@ -0,0 +1,11 @@
+Alternatives: অন্যান্য বিকল্প
+Close: বন্ধ করুন
+Copy: অনুলিপি
+Identifier copied to clipboard!: শনাক্তকারী ক্লিপবোর্ডে অনুলিপি করা হয়েছে!
+Imprint: আইনি তথ্য
+Information: তথ্য
+Language: ভাষা
+Open: খুলুন
+Open Link: লিঙ্ক খুলুন
+Options: বিকল্প
+Warning: সতর্কতা

app/i18n/ui/cs.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternativy
+Close: Zavřít
+Copy: Kopírovat
+Identifier copied to clipboard!: Identifikátor zkopírován do schránky!
+Imprint: Tiráž
+Information: Informace
+Language: Jazyk
+Open: Otevřít
+Open Link: Otevřít odkaz
+Options: Možnosti
+Warning: Upozornění

app/i18n/ui/da.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternativer
+Close: Luk
+Copy: Kopiér
+Identifier copied to clipboard!: Identifikator kopieret til udklipsholder!
+Imprint: Juridisk information
+Information: Information
+Language: Sprog
+Open: Åbn
+Open Link: Åbn link
+Options: Valgmuligheder
+Warning: Advarsel

app/i18n/ui/de.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternativen
+Close: Schließen
+Copy: Kopieren
+Identifier copied to clipboard!: Kennung in die Zwischenablage kopiert!
+Imprint: Impressum
+Information: Information
+Language: Sprache
+Open: Öffnen
+Open Link: Link öffnen
+Options: Optionen
+Warning: Warnung

app/i18n/ui/el.yaml

@@ -0,0 +1,11 @@
+Alternatives: Εναλλακτικές
+Close: Κλείσιμο
+Copy: Αντιγραφή
+Identifier copied to clipboard!: Το αναγνωριστικό αντιγράφηκε στο πρόχειρο!
+Imprint: Νομικές πληροφορίες
+Information: Πληροφορίες
+Language: Γλώσσα
+Open: Άνοιγμα
+Open Link: Άνοιγμα συνδέσμου
+Options: Επιλογές
+Warning: Προειδοποίηση

app/i18n/ui/es.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternativas
+Close: Cerrar
+Copy: Copiar
+Identifier copied to clipboard!: ¡Identificador copiado al portapapeles!
+Imprint: Aviso legal
+Information: Información
+Language: Idioma
+Open: Abrir
+Open Link: Abrir enlace
+Options: Opciones
+Warning: Advertencia

app/i18n/ui/fa.yaml

@@ -0,0 +1,11 @@
+Alternatives: جایگزین‌ها
+Close: بستن
+Copy: کپی
+Identifier copied to clipboard!: شناسه در کلیپ‌بورد کپی شد!
+Imprint: اطلاعات حقوقی
+Information: اطلاعات
+Language: زبان
+Open: باز کردن
+Open Link: باز کردن پیوند
+Options: گزینه‌ها
+Warning: هشدار

app/i18n/ui/fi.yaml

@@ -0,0 +1,11 @@
+Alternatives: Vaihtoehdot
+Close: Sulje
+Copy: Kopioi
+Identifier copied to clipboard!: Tunniste kopioitu leikepöydälle!
+Imprint: Oikeudelliset tiedot
+Information: Tiedot
+Language: Kieli
+Open: Avaa
+Open Link: Avaa linkki
+Options: Valinnat
+Warning: Varoitus

app/i18n/ui/fr.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternatives
+Close: Fermer
+Copy: Copier
+Identifier copied to clipboard!: Identifiant copié dans le presse-papiers !
+Imprint: Mentions légales
+Information: Informations
+Language: Langue
+Open: Ouvrir
+Open Link: Ouvrir le lien
+Options: Options
+Warning: Avertissement

app/i18n/ui/he.yaml

@@ -0,0 +1,11 @@
+Alternatives: חלופות
+Close: סגור
+Copy: העתק
+Identifier copied to clipboard!: המזהה הועתק ללוח!
+Imprint: מידע משפטי
+Information: מידע
+Language: שפה
+Open: פתח
+Open Link: פתח קישור
+Options: אפשרויות
+Warning: אזהרה

app/i18n/ui/hi.yaml

@@ -0,0 +1,11 @@
+Alternatives: अन्य विकल्प
+Close: बंद करें
+Copy: कॉपी करें
+Identifier copied to clipboard!: पहचानकर्ता क्लिपबोर्ड पर कॉपी हो गया!
+Imprint: कानूनी सूचना
+Information: जानकारी
+Language: भाषा
+Open: खोलें
+Open Link: लिंक खोलें
+Options: विकल्प
+Warning: चेतावनी

app/i18n/ui/hu.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternatívák
+Close: Bezárás
+Copy: Másolás
+Identifier copied to clipboard!: Az azonosító a vágólapra másolva!
+Imprint: Impresszum
+Information: Információ
+Language: Nyelv
+Open: Megnyitás
+Open Link: Hivatkozás megnyitása
+Options: Lehetőségek
+Warning: Figyelmeztetés

app/i18n/ui/id.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternatif
+Close: Tutup
+Copy: Salin
+Identifier copied to clipboard!: Pengenal disalin ke papan klip!
+Imprint: Informasi hukum
+Information: Informasi
+Language: Bahasa
+Open: Buka
+Open Link: Buka tautan
+Options: Opsi
+Warning: Peringatan

app/i18n/ui/it.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternative
+Close: Chiudi
+Copy: Copia
+Identifier copied to clipboard!: Identificatore copiato negli appunti!
+Imprint: Note legali
+Information: Informazioni
+Language: Lingua
+Open: Apri
+Open Link: Apri collegamento
+Options: Opzioni
+Warning: Avviso

app/i18n/ui/ja.yaml

@@ -0,0 +1,11 @@
+Alternatives: 代替
+Close: 閉じる
+Copy: コピー
+Identifier copied to clipboard!: 識別子をクリップボードにコピーしました！
+Imprint: 法的情報
+Information: 情報
+Language: 言語
+Open: 開く
+Open Link: リンクを開く
+Options: オプション
+Warning: 警告

app/i18n/ui/ko.yaml

@@ -0,0 +1,11 @@
+Alternatives: 대안
+Close: 닫기
+Copy: 복사
+Identifier copied to clipboard!: 식별자를 클립보드에 복사했습니다!
+Imprint: 법적 고지
+Information: 정보
+Language: 언어
+Open: 열기
+Open Link: 링크 열기
+Options: 옵션
+Warning: 경고

app/i18n/ui/nl.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternatieven
+Close: Sluiten
+Copy: Kopiëren
+Identifier copied to clipboard!: Identificatie gekopieerd naar klembord!
+Imprint: Colofon
+Information: Informatie
+Language: Taal
+Open: Openen
+Open Link: Link openen
+Options: Opties
+Warning: Waarschuwing

app/i18n/ui/pl.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternatywy
+Close: Zamknij
+Copy: Kopiuj
+Identifier copied to clipboard!: Identyfikator skopiowany do schowka!
+Imprint: Nota prawna
+Information: Informacja
+Language: Język
+Open: Otwórz
+Open Link: Otwórz link
+Options: Opcje
+Warning: Ostrzeżenie

app/i18n/ui/pt.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternativas
+Close: Fechar
+Copy: Copiar
+Identifier copied to clipboard!: Identificador copiado para a área de transferência!
+Imprint: Aviso legal
+Information: Informação
+Language: Idioma
+Open: Abrir
+Open Link: Abrir link
+Options: Opções
+Warning: Aviso

app/i18n/ui/ro.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternative
+Close: Închide
+Copy: Copiază
+Identifier copied to clipboard!: Identificator copiat în clipboard!
+Imprint: Notă legală
+Information: Informații
+Language: Limbă
+Open: Deschide
+Open Link: Deschide linkul
+Options: Opțiuni
+Warning: Avertisment

app/i18n/ui/ru.yaml

@@ -0,0 +1,11 @@
+Alternatives: Альтернативы
+Close: Закрыть
+Copy: Копировать
+Identifier copied to clipboard!: Идентификатор скопирован в буфер обмена!
+Imprint: Выходные данные
+Information: Информация
+Language: Язык
+Open: Открыть
+Open Link: Открыть ссылку
+Options: Варианты
+Warning: Предупреждение

app/i18n/ui/sv.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternativ
+Close: Stäng
+Copy: Kopiera
+Identifier copied to clipboard!: Identifierare kopierad till urklipp!
+Imprint: Juridisk information
+Information: Information
+Language: Språk
+Open: Öppna
+Open Link: Öppna länk
+Options: Val
+Warning: Varning

app/i18n/ui/th.yaml

@@ -0,0 +1,11 @@
+Alternatives: ทางเลือกอื่น
+Close: ปิด
+Copy: คัดลอก
+Identifier copied to clipboard!: คัดลอกตัวระบุไปยังคลิปบอร์ดแล้ว!
+Imprint: ข้อมูลทางกฎหมาย
+Information: ข้อมูล
+Language: ภาษา
+Open: เปิด
+Open Link: เปิดลิงก์
+Options: ตัวเลือก
+Warning: คำเตือน

app/i18n/ui/tr.yaml

@@ -0,0 +1,11 @@
+Alternatives: Alternatifler
+Close: Kapat
+Copy: Kopyala
+Identifier copied to clipboard!: Tanımlayıcı panoya kopyalandı!
+Imprint: Künye
+Information: Bilgi
+Language: Dil
+Open: Aç
+Open Link: Bağlantıyı aç
+Options: Seçenekler
+Warning: Uyarı

app/i18n/ui/uk.yaml

@@ -0,0 +1,11 @@
+Alternatives: Альтернативи
+Close: Закрити
+Copy: Копіювати
+Identifier copied to clipboard!: Ідентифікатор скопійовано в буфер обміну!
+Imprint: Вихідні дані
+Information: Інформація
+Language: Мова
+Open: Відкрити
+Open Link: Відкрити посилання
+Options: Варіанти
+Warning: Попередження

app/i18n/ui/ur.yaml

@@ -0,0 +1,11 @@
+Alternatives: متبادل
+Close: بند کریں
+Copy: نقل کریں
+Identifier copied to clipboard!: شناخت کنندہ کلپ بورڈ پر نقل ہو گیا!
+Imprint: قانونی معلومات
+Information: معلومات
+Language: زبان
+Open: کھولیں
+Open Link: لنک کھولیں
+Options: اختیارات
+Warning: انتباہ

app/i18n/ui/vi.yaml

@@ -0,0 +1,11 @@
+Alternatives: Lựa chọn thay thế
+Close: Đóng
+Copy: Sao chép
+Identifier copied to clipboard!: Đã sao chép mã định danh vào bộ nhớ tạm!
+Imprint: Thông tin pháp lý
+Information: Thông tin
+Language: Ngôn ngữ
+Open: Mở
+Open Link: Mở liên kết
+Options: Tùy chọn
+Warning: Cảnh báo

app/i18n/ui/zh.yaml

@@ -0,0 +1,11 @@
+Alternatives: 替代方案
+Close: 关闭
+Copy: 复制
+Identifier copied to clipboard!: 标识符已复制到剪贴板！
+Imprint: 法律声明
+Information: 信息
+Language: 语言
+Open: 打开
+Open Link: 打开链接
+Options: 选项
+Warning: 警告

app/package.json

@@ -7,7 +7,10 @@
     "marked": "^4.3.0"
   },
   "devDependencies": {
-    "cypress": "^14.5.1"
+    "@eslint/js": "^10.0.1",
+    "cypress": "^14.5.1",
+    "eslint": "^10.9.0",
+    "globals": "^17.11.0"
   },
   "scripts": {
     "build": "node scripts/copy-vendor.js",

app/scripts/copy-vendor.js

@@ -29,6 +29,10 @@ copyFile(
   path.join(NM, 'bootstrap', 'dist', 'css', 'bootstrap.min.css'),
   path.join(VENDOR, 'bootstrap', 'css', 'bootstrap.min.css')
 );
+copyFile(
+  path.join(NM, 'bootstrap', 'dist', 'css', 'bootstrap.rtl.min.css'),
+  path.join(VENDOR, 'bootstrap', 'css', 'bootstrap.rtl.min.css')
+);
 copyFile(
   path.join(NM, 'bootstrap', 'dist', 'js', 'bootstrap.bundle.min.js'),
   path.join(VENDOR, 'bootstrap', 'js', 'bootstrap.bundle.min.js')

app/static/css/default.css

@@ -120,6 +120,12 @@ div#navbarNavfooter .navbar-nav {
   flex-wrap: nowrap;
 }
 
+.dropdown-menu.language-menu {
+  max-height: 60vh;
+  overflow-y: auto;
+  overscroll-behavior: contain;
+}
+
 main, footer, header, nav {
   position: relative;
   box-shadow: 

app/static/js/iframe.js

@@ -34,6 +34,10 @@ function syncIframeHeight() {
 
 // Function to open a URL in an iframe (jQuery version mit 1500 ms Fade)
 function openIframe(url) {
+    if (!isSafeUrl(url)) {
+        return;
+    }
+
     var $container    = scrollbarContainer    ? $(scrollbarContainer)    : null;
     var $customScroll = customScrollbar       ? $(customScrollbar)       : null;
     var $main         = $(mainElement);

app/static/js/modal.js

@@ -1,11 +1,48 @@
+function t(source) {
+  return (window.I18N || {})[source] || source;
+}
+
+const SAFE_URL_SCHEMES = ['http:', 'https:', 'mailto:'];
+
+function isSafeUrl(url) {
+  const probe = document.createElement('a');
+  probe.href = String(url == null ? '' : url);
+  return SAFE_URL_SCHEMES.includes(probe.protocol);
+}
+
+function iconAndName(item) {
+  const nodes = [];
+  if (item.icon && item.icon.class) {
+    const icon = document.createElement('i');
+    icon.className = item.icon.class;
+    nodes.push(icon, document.createTextNode(' '));
+  }
+  nodes.push(document.createTextNode(item.name == null ? '' : item.name));
+  return nodes;
+}
+
+function renderMarkdown(content) {
+  const escaped = String(content).replace(/</g, '&lt;').replace(/>/g, '&gt;');
+  const parsed = new DOMParser().parseFromString(marked.parse(escaped), 'text/html');
+
+  parsed.querySelectorAll('a[href]').forEach((anchor) => {
+    if (!SAFE_URL_SCHEMES.includes(anchor.protocol)) {
+      anchor.replaceWith(...anchor.childNodes);
+    }
+  });
+  parsed.querySelectorAll('img[src]').forEach((image) => {
+    if (!SAFE_URL_SCHEMES.includes(image.protocol)) {
+      image.replaceWith(image.alt || '');
+    }
+  });
+
+  return parsed.body.innerHTML;
+}
+
 function openDynamicPopup(subitem) {
   closeAllModals();
   const modalTitle = document.getElementById('dynamicModalLabel');
-  if (subitem.icon && subitem.icon.class) {
-    modalTitle.innerHTML = `<i class="${subitem.icon.class}"></i> ${subitem.name}`;
-  } else {
-    modalTitle.innerText = subitem.name;
-  }
+  modalTitle.replaceChildren(...iconAndName(subitem));
 
   const identifierBox = document.getElementById('dynamicIdentifierBox');
   const modalContent = document.getElementById('dynamicModalContent');
@@ -21,7 +58,7 @@ function openDynamicPopup(subitem) {
     const box = document.getElementById(boxId);
     if (content) {
       box.classList.remove('d-none');
-      document.getElementById(textId).innerHTML = marked.parse(content);
+      document.getElementById(textId).innerHTML = renderMarkdown(content);
     } else {
       box.classList.add('d-none');
     }
@@ -44,16 +81,19 @@ function openDynamicPopup(subitem) {
   if (subitem.url) {
     linkBox.classList.remove('d-none');
     linkHref.href = subitem.url;
-    linkHref.innerText = subitem.description || "Open Link";
+    if (!isSafeUrl(subitem.url)) {
+      linkHref.removeAttribute('href');
+    }
+    linkHref.innerText = subitem.description || t("Open Link");
+    linkHref.classList.remove('iframe');
+    linkHref.onclick = null;
     if (subitem.iframe) {
       linkHref.classList.add('iframe');
-      // Attach an event listener that prevents the default behavior and
-      // opens the URL in an iframe when clicked.
-      linkHref.addEventListener('click', function(event) {
+      linkHref.onclick = function(event) {
         event.preventDefault();
         openIframe(subitem.url);
-        closeAllModals()
-      });
+        closeAllModals();
+      };
     }
   } else {
     linkBox.classList.add('d-none');
@@ -69,13 +109,13 @@ function openDynamicPopup(subitem) {
       items.forEach(item => {
         const listItem = document.createElement('li');
         listItem.classList.add('list-group-item', 'd-flex', 'justify-content-between', 'align-items-center');
-        listItem.innerHTML = `
-          <span>
-            <i class="${item.icon.class}"></i> ${item.name}
-          </span>
-          <button class="btn btn-outline-secondary btn-sm">Open</button>
-        `;
-        listItem.querySelector('button').addEventListener('click', () => onClickHandler(item));
+        const label = document.createElement('span');
+        label.replaceChildren(...iconAndName(item));
+        const button = document.createElement('button');
+        button.className = 'btn btn-outline-secondary btn-sm';
+        button.textContent = t('Open');
+        listItem.replaceChildren(label, button);
+        button.addEventListener('click', () => onClickHandler(item));
         list.appendChild(listItem);
       });
     } else {
@@ -90,7 +130,7 @@ function openDynamicPopup(subitem) {
   copyButton.onclick = () => {
     modalContent.select();
     navigator.clipboard.writeText(modalContent.value).then(() => {
-      alert('Identifier copied to clipboard!');
+      alert(t('Identifier copied to clipboard!'));
     });
   };
 

app/static/js/navigation.js

@@ -169,7 +169,6 @@ document.addEventListener('DOMContentLoaded', () => {
   }
     
   function adjustMenuPosition(submenu, parent, isTopLevel) {
-    const rect = submenu.getBoundingClientRect();
     const parentRect = parent.getBoundingClientRect();
 
     const spaceAbove = parentRect.top;

app/templates/moduls/base.html.j2

@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html lang="en">
+<html lang="{{ lang }}" dir="{{ lang_dir }}">
     <head>
         <title>{{platform.titel}}</title>
         <meta charset="utf-8" >
@@ -8,8 +8,13 @@
             type="image/x-icon"
             href="{{ asset_src(platform.favicon) }}"
         >
+        <link rel="canonical" href="{{ url_for('localized_index', lang=lang, _external=True) }}">
+        {% for code in languages %}
+        <link rel="alternate" hreflang="{{ code }}" href="{{ url_for('localized_index', lang=code, _external=True) }}">
+        {% endfor %}
+        <link rel="alternate" hreflang="x-default" href="{{ url_for('index', _external=True) }}">
         <!-- Bootstrap CSS only -->
-        <link href="{{ url_for('static', filename='vendor/bootstrap/css/bootstrap.min.css') }}" rel="stylesheet">
+        <link href="{{ url_for('static', filename='vendor/bootstrap/css/bootstrap' ~ ('.rtl' if lang_dir == 'rtl' else '') ~ '.min.css') }}" rel="stylesheet">
         <!-- Bootstrap JavaScript Bundle with Popper -->
         <script src="{{ url_for('static', filename='vendor/bootstrap/js/bootstrap.bundle.min.js') }}"></script>
         <!-- Bootstrap Icons -->
@@ -60,12 +65,13 @@
                     <p itemprop="name">{{ company.titel }} <br />
                     {{ company.subtitel }}</p>
                     <span><i class="fa-solid fa-location-dot"></i> {{ company.address.values() | join(", ") }}</span>
-                    <p><a href="{{company.imprint_url}}" class="iframe-link"><i class="fa-solid fa-scale-balanced"></i> Imprint</a></p>
+                    <p><a href="{{company.imprint_url}}" class="iframe-link"><i class="fa-solid fa-scale-balanced"></i> {{ t('Imprint') }}</a></p>
                 </div>
             </footer>
         </div>
         <!-- Include modal -->
         {% include "moduls/modal.html.j2" %}
+        <script>window.I18N = {{ ui_strings | tojson }};</script>
         {% for name in [
             'modal',
             'navigation',

app/templates/moduls/modal.html.j2

@@ -16,14 +16,14 @@
     <div class="modal-content">
       <div class="modal-header">
         <h5 class="modal-title" id="dynamicModalLabel"></h5>
-        <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
+        <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="{{ t('Close') }}"></button>
       </div>
       <div class="modal-body">
         <!-- Warning box with Markdown -->
-        {{ alert_box('dynamicModalWarning', 'alert-warning', 'fa-solid fa-triangle-exclamation', 'Warning', 'dynamicModalWarningText') }}
+        {{ alert_box('dynamicModalWarning', 'alert-warning', 'fa-solid fa-triangle-exclamation', t('Warning'), 'dynamicModalWarningText') }}
 
         <!-- Info box with Markdown -->
-        {{ alert_box('dynamicModalInfo', 'alert-info', 'fa-solid fa-circle-info', 'Information', 'dynamicModalInfoText') }}
+        {{ alert_box('dynamicModalInfo', 'alert-info', 'fa-solid fa-circle-info', t('Information'), 'dynamicModalInfoText') }}
 
         <!-- Description text -->
         <div id="dynamicDescriptionText" class="mt-2 d-none"></div>
@@ -31,7 +31,7 @@
         <!-- Input box for Identifier -->
         <div id="dynamicIdentifierBox" class="input-group mt-2 d-none">
           <input type="text" id="dynamicModalContent" class="form-control" readonly>
-          <button class="btn btn-outline-secondary" type="button" id="dynamicCopyButton">Copy</button>
+          <button class="btn btn-outline-secondary" type="button" id="dynamicCopyButton">{{ t('Copy') }}</button>
         </div>
 
         <!-- Link -->
@@ -40,13 +40,13 @@
         </div>
 
         <!-- Options -->
-        {{ list_section('dynamicChildrenSection', 'Options', 'dynamicChildrenList') }}
+        {{ list_section('dynamicChildrenSection', t('Options'), 'dynamicChildrenList') }}
 
         <!-- Alternatives -->
-        {{ list_section('dynamicAlternativesSection', 'Alternatives', 'dynamicAlternativesList') }}
+        {{ list_section('dynamicAlternativesSection', t('Alternatives'), 'dynamicAlternativesList') }}
       </div>
       <div class="modal-footer">
-        <button type="button" class="btn btn-secondary" data-bs-dismiss="modal" onclick="closeAllModals()">Close</button>
+        <button type="button" class="btn btn-secondary" data-bs-dismiss="modal" onclick="closeAllModals()">{{ t('Close') }}</button>
       </div>
     </div>
   </div>

app/templates/moduls/navigation.html.j2

@@ -98,6 +98,23 @@
                     </li>
                 {% endif %}
             {% endfor %}
+            {% if menu_type == "header" %}
+            <li class="nav-item dropdown">
+                <a class="nav-link dropdown-toggle btn btn-light" id="navbarDropdownLanguage" role="button" data-bs-toggle="dropdown" data-bs-display="dynamic" aria-expanded="false" title="{{ t('Language') }}">
+                    <i class="fa-solid fa-language"></i> {{ languages[lang] }}
+                </a>
+                <ul class="dropdown-menu language-menu">
+                    {% for code, endonym in languages.items() %}
+                    <li>
+                        <a class="dropdown-item {% if code == lang %}active{% endif %}"
+                           href="{{ url_for('localized_index', lang=code) }}"
+                           hreflang="{{ code }}"
+                           lang="{{ code }}">{{ endonym }}</a>
+                    </li>
+                    {% endfor %}
+                </ul>
+            </li>
+            {% endif %}
         </ul>
     </div>
 </nav>

app/utils/i18n.py

@@ -0,0 +1,134 @@
+"""Language negotiation and translation of the resolved configuration tree.
+
+Translation is catalogue-driven: a string is replaced only when the target
+language's catalogue holds an entry for the exact English source string.
+Anything unknown falls through to English, so a partially filled catalogue
+degrades instead of breaking.
+"""
+
+import logging
+from pathlib import Path
+
+import yaml
+
+try:
+    from app.utils.languages import LANGUAGES, RTL_LANGUAGES
+except ImportError:  # pragma: no cover - supports running from the app/ directory.
+    from utils.languages import LANGUAGES, RTL_LANGUAGES
+
+I18N_DIR = Path(__file__).resolve().parent.parent / "i18n"
+UI_DIR = I18N_DIR / "ui"
+CONTENT_DIR = I18N_DIR / "content"
+
+SOURCE_LANGUAGE = "en"
+
+
+TRANSLATABLE_KEYS = frozenset(
+    {"description", "info", "name", "subtitel", "text", "title", "warning"}
+)
+
+UI_STRINGS = (
+    "Alternatives",
+    "Close",
+    "Copy",
+    "Identifier copied to clipboard!",
+    "Imprint",
+    "Information",
+    "Language",
+    "Open",
+    "Open Link",
+    "Options",
+    "Warning",
+)
+
+_catalogs: dict[str, dict[str, str]] = {}
+
+
+def direction(code):
+    """Return the writing direction of ``code`` as an HTML ``dir`` value."""
+    return "rtl" if code in RTL_LANGUAGES else "ltr"
+
+
+def read_catalog(path):
+    """Return the catalogue at ``path``, or an empty one if it is unusable.
+
+    Catalogues are hand-edited and machine-written, so a stray character must
+    degrade that language to English rather than take every page down with a
+    parse error. Non-string entries are dropped for the same reason: they would
+    otherwise reach the templates and render as ``42`` or ``null``.
+    """
+    if not path.exists():
+        return {}
+    try:
+        loaded = yaml.safe_load(path.read_text(encoding="utf-8"))
+    except (OSError, UnicodeDecodeError, yaml.YAMLError):
+        logging.warning("Ignoring unreadable translation catalogue: %s", path)
+        return {}
+    if not isinstance(loaded, dict):
+        logging.warning(
+            "Ignoring translation catalogue that is not a mapping: %s", path
+        )
+        return {}
+    return {
+        key: value
+        for key, value in loaded.items()
+        if isinstance(key, str) and isinstance(value, str)
+    }
+
+
+def clear_catalogs():
+    """Drop the memoized catalogues so edited files are picked up."""
+    _catalogs.clear()
+
+
+def catalog(code):
+    """Return the merged UI and content catalogue for ``code``."""
+    if code not in _catalogs:
+        _catalogs[code] = {
+            **read_catalog(UI_DIR / f"{code}.yaml"),
+            **read_catalog(CONTENT_DIR / f"{code}.yaml"),
+        }
+    return _catalogs[code]
+
+
+def negotiate(accepted, default=SOURCE_LANGUAGE):
+    """Pick the best supported language from ``Accept-Language`` pairs.
+
+    Args:
+        accepted: iterable of ``(tag, quality)`` as produced by
+            ``flask.request.accept_languages``.
+        default: language returned when no tag is supported.
+
+    Werkzeug's own ``best_match`` returns an exact match before it considers
+    primary-tag fallbacks, so ``de-DE,en;q=0.8`` resolves to English. Matching
+    on the primary subtag up front avoids that.
+    """
+    best, best_quality = default, 0.0
+    for tag, quality in accepted:
+        code = tag.replace("_", "-").split("-")[0].lower()
+        if code in LANGUAGES and quality > best_quality:
+            best, best_quality = code, quality
+    return best
+
+
+def translate_tree(node, code, key=None):
+    """Return a copy of ``node`` with translatable leaves swapped for ``code``.
+
+    Args:
+        node: the resolved configuration tree, or any subtree of it.
+        code: target language code.
+        key: the mapping key ``node`` was reached through.
+    """
+    if isinstance(node, dict):
+        return {name: translate_tree(value, code, name) for name, value in node.items()}
+    if isinstance(node, list):
+        return [translate_tree(item, code, key) for item in node]
+    if isinstance(node, str) and key in TRANSLATABLE_KEYS:
+        return catalog(code).get(node, node)
+    return node
+
+
+def ui_strings(code):
+    """Return the interface strings for ``code``, keyed by their English source."""
+    entries = catalog(code)
+    return {source: entries.get(source, source) for source in UI_STRINGS}

app/utils/languages.py

@@ -0,0 +1,196 @@
+"""ISO 639-1 languages, their display names and writing direction.
+
+Generated by utils/generate_languages.py — edit that script, not this file.
+
+Display names are CLDR endonyms where CLDR covers the language and the English
+ISO 639-1 name for the 25 codes it does not.
+"""
+
+LANGUAGES = {
+    "en": "English",
+    "aa": "Qafar",
+    "ab": "Аԥсшәа",
+    "ae": "Avestan",
+    "af": "Afrikaans",
+    "ak": "Akan",
+    "am": "አማርኛ",
+    "an": "aragonés",
+    "ar": "العربية",
+    "as": "অসমীয়া",
+    "av": "Avaric",
+    "ay": "Aymara",
+    "az": "azərbaycan",
+    "ba": "башҡорт теле",
+    "be": "беларуская",
+    "bg": "български",
+    "bi": "Bislama",
+    "bm": "bamanakan",
+    "bn": "বাংলা",
+    "bo": "བོད་སྐད་",
+    "br": "brezhoneg",
+    "bs": "bosanski",
+    "ca": "català",
+    "ce": "нохчийн",
+    "ch": "Chamorro",
+    "co": "corsu",
+    "cr": "Cree",
+    "cs": "čeština",
+    "cu": "церковнослове́нскїй",
+    "cv": "чӑваш",
+    "cy": "Cymraeg",
+    "da": "dansk",
+    "de": "Deutsch",
+    "dv": "ދިވެހިބަސް",
+    "dz": "རྫོང་ཁ",
+    "ee": "eʋegbe",
+    "el": "Ελληνικά",
+    "eo": "Esperanto",
+    "es": "español",
+    "et": "eesti",
+    "eu": "euskara",
+    "fa": "فارسی",
+    "ff": "Pulaar",
+    "fi": "suomi",
+    "fj": "Fijian",
+    "fo": "føroyskt",
+    "fr": "français",
+    "fy": "Frysk",
+    "ga": "Gaeilge",
+    "gd": "Gàidhlig",
+    "gl": "galego",
+    "gn": "avañe’ẽ",
+    "gu": "ગુજરાતી",
+    "gv": "Gaelg",
+    "ha": "Hausa",
+    "he": "עברית",
+    "hi": "हिन्दी",
+    "ho": "Hiri Motu",
+    "hr": "hrvatski",
+    "ht": "Kreyòl Ayisyen",
+    "hu": "magyar",
+    "hy": "հայերեն",
+    "hz": "Herero",
+    "ia": "interlingua",
+    "id": "Indonesia",
+    "ie": "Interlingue",
+    "ig": "Igbo",
+    "ii": "ꆈꌠꉙ",
+    "ik": "Inupiaq",
+    "io": "Ido",
+    "is": "íslenska",
+    "it": "italiano",
+    "iu": "ᐃᓄᒃᑎᑐᑦ",
+    "ja": "日本語",
+    "jv": "Jawa",
+    "ka": "ქართული",
+    "kg": "Kongo",
+    "ki": "Gikuyu",
+    "kj": "Kuanyama",
+    "kk": "қазақ тілі",
+    "kl": "kalaallisut",
+    "km": "ខ្មែរ",
+    "kn": "ಕನ್ನಡ",
+    "ko": "한국어",
+    "kr": "Kanuri",
+    "ks": "کٲشُر",
+    "ku": "kurdî (kurmancî)",
+    "kv": "Komi",
+    "kw": "kernewek",
+    "ky": "кыргызча",
+    "la": "Latina",
+    "lb": "Lëtzebuergesch",
+    "lg": "Luganda",
+    "li": "Limburgan",
+    "ln": "lingála",
+    "lo": "ລາວ",
+    "lt": "lietuvių",
+    "lu": "Tshiluba",
+    "lv": "latviešu",
+    "mg": "Malagasy",
+    "mh": "Marshallese",
+    "mi": "Māori",
+    "mk": "македонски",
+    "ml": "മലയാളം",
+    "mn": "монгол",
+    "mr": "मराठी",
+    "ms": "Melayu",
+    "mt": "Malti",
+    "my": "မြန်မာ",
+    "na": "Nauru",
+    "nb": "norsk bokmål",
+    "nd": "isiNdebele",
+    "ne": "नेपाली",
+    "ng": "Ndonga",
+    "nl": "Nederlands",
+    "nn": "norsk nynorsk",
+    "no": "norsk",
+    "nr": "isiNdebele",
+    "nv": "Diné Bizaad",
+    "ny": "Nyanja",
+    "oc": "occitan",
+    "oj": "Ojibwa",
+    "om": "Oromoo",
+    "or": "ଓଡ଼ିଆ",
+    "os": "ирон",
+    "pa": "ਪੰਜਾਬੀ",
+    "pi": "Pali",
+    "pl": "polski",
+    "ps": "پښتو",
+    "pt": "português",
+    "qu": "Runasimi",
+    "rm": "rumantsch",
+    "rn": "Ikirundi",
+    "ro": "română",
+    "ru": "русский",
+    "rw": "Ikinyarwanda",
+    "sa": "संस्कृत भाषा",
+    "sc": "sardu",
+    "sd": "سنڌي",
+    "se": "davvisámegiella",
+    "sg": "Sängö",
+    "sh": "Serbo-Croatian",
+    "si": "සිංහල",
+    "sk": "slovenčina",
+    "sl": "slovenščina",
+    "sm": "Samoan",
+    "sn": "chiShona",
+    "so": "Soomaali",
+    "sq": "shqip",
+    "sr": "српски",
+    "ss": "siSwati",
+    "st": "Sesotho",
+    "su": "Basa Sunda",
+    "sv": "svenska",
+    "sw": "Kiswahili",
+    "ta": "தமிழ்",
+    "te": "తెలుగు",
+    "tg": "тоҷикӣ",
+    "th": "ไทย",
+    "ti": "ትግርኛ",
+    "tk": "türkmen dili",
+    "tl": "Tagalog",
+    "tn": "Setswana",
+    "to": "lea fakatonga",
+    "tr": "Türkçe",
+    "ts": "Xitsonga",
+    "tt": "татар",
+    "tw": "Twi",
+    "ty": "Tahitian",
+    "ug": "ئۇيغۇرچە",
+    "uk": "українська",
+    "ur": "اردو",
+    "uz": "o‘zbek",
+    "ve": "Tshivenḓa",
+    "vi": "Tiếng Việt",
+    "vo": "Volapük",
+    "wa": "walon",
+    "wo": "Wolof",
+    "xh": "IsiXhosa",
+    "yi": "ייִדיש",
+    "yo": "Èdè Yorùbá",
+    "za": "Vahcuengh",
+    "zh": "中文",
+    "zu": "isiZulu",
+}
+
+RTL_LANGUAGES = frozenset({"ar", "dv", "fa", "he", "ks", "ps", "sd", "ug", "ur", "yi"})

env.example

@@ -4,3 +4,10 @@ FLASK_ENV=production
 # Makefile (build/run-dev/run-prod) and docker-compose.yml so every
 # build path produces and consumes the same tag.
 IMAGE_NAME=portfolio
+# Comma-separated public hostnames. Leave empty and the app reflects whatever
+# Host header arrives into its canonical, hreflang and redirect URLs.
+TRUSTED_HOSTS=
+# LibreTranslate instance used by `make i18n` to fill app/i18n/content/.
+# Only needed when generating translations, never at runtime.
+LIBRETRANSLATE_URL=http://localhost:5002
+LIBRETRANSLATE_API_KEY=

pyproject.toml

@@ -4,21 +4,26 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "portfolio-ui"
-version = "2.0.0"
+version = "2.1.0"
 description = "A lightweight YAML-driven portfolio and landing-page generator."
 readme = "README.md"
 requires-python = ">=3.12"
 dependencies = [
-    "flask",
+    # 3.1 introduced TRUSTED_HOSTS; an older Flask accepts the config key and
+    # ignores it, which would silently disable the host-forgery check.
+    "flask>=3.1",
     "pyyaml",
     "requests",
 ]
 
 [project.optional-dependencies]
 dev = [
+    "babel",
     "bandit",
     "pip-audit",
+    "pycountry",
     "ruff",
+    "yamllint",
 ]
 
 [tool.setuptools]
@@ -30,6 +35,8 @@ include = ["app", "app.*"]
 [tool.setuptools.package-data]
 app = [
     "config.sample.yaml",
+    "i18n/ui/*.yaml",
+    "i18n/content/*.yaml",
     "templates/**/*.j2",
     "static/css/*.css",
     "static/js/*.js",

scripts/run-e2e.sh

@@ -0,0 +1,73 @@
+#!/bin/sh
+# Run the Cypress suite against a Flask app started by this script.
+#
+# Server and browser share one shell, so this works where act cannot reach the
+# host network. ELECTRON_RUN_AS_NODE is dropped before invoking Cypress because
+# VS Code exports it to child processes, which makes Cypress' bundled Electron
+# behave as Node and reject its own launch flags.
+#
+# Env:
+#   PORT     required, the port Flask binds and Cypress targets
+#   PYTHON   interpreter to run app.py with (default: python3)
+#   E2E_LOG  where Flask's output goes (default: /tmp/portfolio-e2e-flask.log)
+set -u
+
+PORT="${PORT:?PORT must be set (see env.example)}"
+PYTHON="${PYTHON:-python3}"
+E2E_LOG="${E2E_LOG:-/tmp/portfolio-e2e-flask.log}"
+FLASK_HOST=127.0.0.1
+export PORT FLASK_HOST
+
+APP_DIR="$(CDPATH='' cd -- "$(dirname -- "$0")/../app" && pwd)"
+BASE_URL="http://127.0.0.1:$PORT/"
+
+# cypress.config.js defaults to localhost, which resolves to ::1 first on a
+# dual-stack host — a different listener from the 127.0.0.1 one Flask binds and
+# this script probes. Pin all three to the same origin.
+export CYPRESS_baseUrl="$BASE_URL"
+
+# A foreign listener would be tested instead of the working tree. The predicate
+# is "something answers", deliberately without curl's -f: a foreign server that
+# is still returning 5xx right now would otherwise pass the guard and then be
+# picked up by the readiness probe once it recovers. --noproxy is what keeps
+# that same looser predicate from matching an HTTP proxy instead of the port.
+if curl -s --noproxy '*' --connect-timeout 2 --max-time 10 -o /dev/null "$BASE_URL"; then
+    echo "ERROR: something already serves port $PORT — Cypress would test that"
+    echo "       instead of your working tree. Stop it first ('make down' for"
+    echo "       the container, otherwise a stray 'python app.py')."
+    exit 1
+fi
+
+cd "$APP_DIR" || exit 1
+$PYTHON app.py > "$E2E_LOG" 2>&1 &
+flask_pid=$!
+trap 'kill "$flask_pid" 2>/dev/null || true' EXIT INT TERM
+
+echo "Waiting for $BASE_URL — follow with: tail -f $E2E_LOG"
+attempt=0
+while [ "$attempt" -lt 120 ]; do
+    # Liveness first: a server answering while ours is dead means we are about
+    # to hand Cypress somebody else's app.
+    if ! kill -0 "$flask_pid" 2>/dev/null; then
+        echo "ERROR: Flask exited during startup"
+        cat "$E2E_LOG"
+        exit 1
+    fi
+    if curl -sf --noproxy '*' --connect-timeout 2 --max-time 10 -o /dev/null "$BASE_URL"; then
+        break
+    fi
+    attempt=$((attempt + 1))
+    sleep 1
+done
+
+if ! curl -sf --noproxy '*' --connect-timeout 2 --max-time 10 -o /dev/null "$BASE_URL"; then
+    echo "ERROR: app never became ready"
+    cat "$E2E_LOG"
+    exit 1
+fi
+
+env -u ELECTRON_RUN_AS_NODE npx cypress run
+status=$?
+
+kill "$flask_pid" 2>/dev/null || true
+exit "$status"

tests/integration/test_app_routes.py

@@ -0,0 +1,258 @@
+import json
+import os
+import shutil
+import subprocess
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+from unittest.mock import Mock, patch
+
+import requests
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+
+app_module = None
+flask_app = None
+i18n = None
+_origin = None
+_workdir = None
+
+
+def setUpModule():
+    """Import the app with a disposable configuration as the working directory.
+
+    ``app.app`` reads ``config.yaml`` relative to the process working directory
+    at import time, so the chdir has to happen before the import.
+    """
+    global app_module, flask_app, i18n, _origin, _workdir
+
+    _origin = os.getcwd()
+    _workdir = tempfile.mkdtemp(prefix="portfolio-routes-")
+    shutil.copy(
+        REPO_ROOT / "app" / "config.sample.yaml", Path(_workdir) / "config.yaml"
+    )
+    os.chdir(_workdir)
+
+    from app import app as imported_module
+    from app.utils import i18n as imported_i18n
+
+    app_module = imported_module
+    flask_app = imported_module.app
+    i18n = imported_i18n
+    flask_app.config["NASA_API_KEY"] = None
+
+
+def tearDownModule():
+    os.chdir(_origin)
+    shutil.rmtree(_workdir, ignore_errors=True)
+
+
+class AppRouteMixin:
+    """Shared setup. A mixin rather than a TestCase subclass, so that every
+    test class below still names ``unittest.TestCase`` as a direct base — the
+    lint guardrail in tests/lint/ does not resolve inherited aliases."""
+
+    def setUp(self):
+        self.client = flask_app.test_client()
+        self.addCleanup(i18n.clear_catalogs)
+        self.addCleanup(flask_app.config["TRANSLATED_CONFIG"].clear)
+
+
+class TestRouting(AppRouteMixin, unittest.TestCase):
+    def test_unrelated_single_segment_paths_are_not_redirected(self):
+        for path in ("/robots.txt", "/favicon.ico", "/sitemap.xml"):
+            with self.subTest(path=path):
+                self.assertEqual(self.client.get(path).status_code, 404)
+
+    def test_language_path_redirects_to_the_canonical_trailing_slash(self):
+        response = self.client.get("/de")
+
+        self.assertEqual(response.status_code, 308)
+        self.assertTrue(response.headers["Location"].endswith("/de/"))
+
+    def test_supported_language_renders(self):
+        response = self.client.get("/de/")
+
+        self.assertEqual(response.status_code, 200)
+        self.assertIn('<html lang="de"', response.get_data(as_text=True))
+
+    def test_unsupported_language_is_not_found(self):
+        self.assertEqual(self.client.get("/xx/").status_code, 404)
+
+
+class TestNegotiation(AppRouteMixin, unittest.TestCase):
+    def test_regional_tag_beats_a_lower_ranked_exact_match(self):
+        response = self.client.get("/", headers={"Accept-Language": "de-DE,en;q=0.8"})
+
+        self.assertIn('<html lang="de"', response.get_data(as_text=True))
+
+    def test_negotiated_route_declares_that_it_varies(self):
+        response = self.client.get("/", headers={"Accept-Language": "de-DE"})
+
+        self.assertEqual(response.headers.get("Vary"), "Accept-Language")
+
+
+class TestEscaping(AppRouteMixin, unittest.TestCase):
+    def test_catalog_content_is_html_escaped(self):
+        i18n._catalogs["de"] = {"Copy": "<script>alert('ui')</script>"}
+
+        body = self.client.get("/de/").get_data(as_text=True)
+
+        self.assertNotIn("<script>alert('ui')</script>", body)
+        self.assertIn("&lt;script&gt;alert(&#39;ui&#39;)", body)
+
+    def test_configuration_content_is_html_escaped(self):
+        i18n._catalogs["de"] = {"Imprint": "<script>alert('config')</script>"}
+
+        body = self.client.get("/de/").get_data(as_text=True)
+
+        self.assertNotIn("<script>alert('config')</script>", body)
+        self.assertIn("&lt;script&gt;alert(&#39;config&#39;)", body)
+
+
+class TestApodBackground(AppRouteMixin, unittest.TestCase):
+    def setUp(self):
+        super().setUp()
+        self.addCleanup(flask_app.config.__setitem__, "NASA_API_KEY", None)
+        flask_app.config["NASA_API_KEY"] = "key"
+
+    def test_no_request_is_made_without_a_key(self):
+        flask_app.config["NASA_API_KEY"] = None
+
+        with patch("app.app.requests.get") as get:
+            self.assertIsNone(app_module.apod_background())
+
+        get.assert_not_called()
+
+    def test_a_transport_failure_costs_the_background_not_the_page(self):
+        with patch(
+            "app.app.requests.get", side_effect=requests.ConnectionError("down")
+        ):
+            self.assertIsNone(app_module.apod_background())
+
+        self.assertEqual(self.client.get("/en/").status_code, 200)
+
+    def test_an_error_response_costs_the_background_not_the_page(self):
+        refusal = Mock(ok=False)
+        refusal.json.return_value = {"media_type": "image", "url": "https://i.test/x"}
+
+        with patch("app.app.requests.get", return_value=refusal):
+            self.assertIsNone(app_module.apod_background())
+
+    def test_a_video_of_the_day_is_not_used_as_a_background(self):
+        answer = Mock(ok=True)
+        answer.json.return_value = {"media_type": "video", "url": "https://v.test/x"}
+
+        with patch("app.app.requests.get", return_value=answer):
+            self.assertIsNone(app_module.apod_background())
+
+    def test_an_image_of_the_day_is_used(self):
+        answer = Mock(ok=True)
+        answer.json.return_value = {"media_type": "image", "url": "https://i.test/x"}
+
+        with patch("app.app.requests.get", return_value=answer):
+            self.assertEqual(app_module.apod_background(), "https://i.test/x")
+
+
+class TestExternalUrls(AppRouteMixin, unittest.TestCase):
+    def test_only_the_forwarded_scheme_is_trusted(self):
+        proxy = flask_app.wsgi_app
+
+        self.assertEqual(proxy.x_proto, 1)
+        self.assertEqual(
+            (proxy.x_for, proxy.x_host, proxy.x_port, proxy.x_prefix), (0, 0, 0, 0)
+        )
+
+    def test_forwarded_scheme_reaches_the_canonical_and_alternates(self):
+        body = self.client.get(
+            "/en/",
+            headers={"Host": "portfolio.example.org", "X-Forwarded-Proto": "https"},
+        ).get_data(as_text=True)
+
+        self.assertIn(
+            '<link rel="canonical" href="https://portfolio.example.org/en/">', body
+        )
+        self.assertIn('hreflang="ja" href="https://portfolio.example.org/ja/"', body)
+        self.assertNotIn("http://portfolio.example.org", body)
+
+    def test_trusted_hosts_are_parsed_from_a_comma_separated_list(self):
+        self.assertEqual(
+            app_module.trusted_hosts("a.test, b.test ,"), ["a.test", "b.test"]
+        )
+
+    def test_an_unset_trusted_hosts_value_disables_the_check(self):
+        self.assertIsNone(app_module.trusted_hosts(""))
+        self.assertIsNone(app_module.trusted_hosts("  ,  "))
+
+    def test_a_forged_host_is_rejected_once_trusted_hosts_are_named(self):
+        self.addCleanup(flask_app.config.__setitem__, "TRUSTED_HOSTS", None)
+        flask_app.config["TRUSTED_HOSTS"] = ["portfolio.example.org"]
+
+        forged = self.client.get("/de", headers={"Host": "evil.test"})
+        honest = self.client.get("/de/", headers={"Host": "portfolio.example.org"})
+
+        self.assertEqual(forged.status_code, 400)
+        self.assertEqual(honest.status_code, 200)
+
+    def test_forwarded_host_is_not_trusted(self):
+        response = self.client.get(
+            "/de",
+            headers={"Host": "portfolio.example.org", "X-Forwarded-Host": "evil.test"},
+        )
+
+        self.assertNotIn("evil.test", response.headers["Location"])
+
+        body = self.client.get(
+            "/en/",
+            headers={"Host": "portfolio.example.org", "X-Forwarded-Host": "evil.test"},
+        ).get_data(as_text=True)
+
+        self.assertNotIn("evil.test", body)
+
+
+class TestConfigurationReload(AppRouteMixin, unittest.TestCase):
+    def test_reloading_the_configuration_drops_the_catalog_memo(self):
+        i18n.catalog("de")
+        self.assertIn("de", i18n._catalogs)
+
+        app_module.load_config(flask_app)
+
+        self.assertEqual(i18n._catalogs, {})
+
+    def test_reloading_the_configuration_drops_the_translation_memo(self):
+        self.client.get("/de/")
+        self.assertIn("de", flask_app.config["TRANSLATED_CONFIG"])
+
+        app_module.load_config(flask_app)
+
+        self.assertEqual(flask_app.config["TRANSLATED_CONFIG"], {})
+
+
+class TestTrustedHostsWiring(unittest.TestCase):
+    def test_the_environment_reaches_the_flask_configuration(self):
+        script = (
+            "import json, sys;"
+            f"sys.path.insert(0, {str(REPO_ROOT)!r});"
+            "from app.app import app;"
+            "print('TRUSTED=' + json.dumps(app.config['TRUSTED_HOSTS']))"
+        )
+        result = subprocess.run(
+            [sys.executable, "-c", script],
+            cwd=_workdir,
+            env={**os.environ, "TRUSTED_HOSTS": "a.test, b.test"},
+            capture_output=True,
+            text=True,
+            check=True,
+        )
+
+        reported = [
+            line for line in result.stdout.splitlines() if line.startswith("TRUSTED=")
+        ]
+        self.assertEqual(
+            json.loads(reported[-1][len("TRUSTED=") :]), ["a.test", "b.test"]
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/integration/test_python_packaging.py

@@ -1,8 +1,17 @@
+import re
 import tomllib
 import unittest
 from pathlib import Path
 
 
+def distributions(requirements):
+    """Return the distribution names of PEP 508 requirement strings."""
+    return {
+        re.split(r"[<>=!~\[; ]", requirement, maxsplit=1)[0]
+        for requirement in requirements
+    }
+
+
 class TestPythonPackaging(unittest.TestCase):
     def setUp(self) -> None:
         self.repo_root = Path(__file__).resolve().parents[2]
@@ -18,11 +27,20 @@ class TestPythonPackaging(unittest.TestCase):
         self.assertEqual(build_system["build-backend"], "setuptools.build_meta")
         self.assertIn("setuptools>=69", build_system["requires"])
         self.assertGreaterEqual(
-            set(project["dependencies"]),
+            distributions(project["dependencies"]),
             {"flask", "pyyaml", "requests"},
         )
         self.assertEqual(project["requires-python"], ">=3.12")
 
+    def test_flask_is_pinned_to_a_version_that_honours_trusted_hosts(self):
+        requirement = next(
+            item
+            for item in self.pyproject["project"]["dependencies"]
+            if item.startswith("flask")
+        )
+
+        self.assertIn(">=3.1", requirement)
+
     def test_pyproject_defines_dev_dependencies_and_package_contents(self):
         project = self.pyproject["project"]
         setuptools_config = self.pyproject["tool"]["setuptools"]
@@ -30,8 +48,8 @@ class TestPythonPackaging(unittest.TestCase):
         package_data = setuptools_config["package-data"]["app"]
 
         self.assertGreaterEqual(
-            set(project["optional-dependencies"]["dev"]),
-            {"bandit", "pip-audit", "ruff"},
+            distributions(project["optional-dependencies"]["dev"]),
+            {"bandit", "pip-audit", "ruff", "yamllint"},
         )
         self.assertEqual(setuptools_config["py-modules"], ["main"])
         self.assertEqual(package_find["include"], ["app", "app.*"])

tests/lint/test_tooling_configuration.py

@@ -0,0 +1,157 @@
+"""Guards for configuration that no other test observes.
+
+Each assertion here stands for a defect that was found by deleting the line it
+checks: the deletion is invisible to every suite, and its effect only shows up
+in production or in a fresh checkout.
+"""
+
+import json
+import re
+import tomllib
+import unittest
+from pathlib import Path
+
+import yaml
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+
+
+class TestYamllintConfiguration(unittest.TestCase):
+    def setUp(self):
+        self.config = yaml.safe_load(
+            (REPO_ROOT / ".yamllint").read_text(encoding="utf-8")
+        )
+
+    def test_duplicate_keys_are_an_error(self):
+        self.assertEqual(self.config["rules"]["key-duplicates"], "enable")
+
+    def test_the_directories_that_collect_foreign_yaml_are_ignored(self):
+        ignored = self.config["ignore"].split()
+
+        self.assertGreaterEqual(
+            set(ignored),
+            {".git/", ".venv/", "node_modules/", "app/node_modules/"},
+        )
+
+
+class TestRunTargets(unittest.TestCase):
+    def setUp(self):
+        self.makefile = (REPO_ROOT / "Makefile").read_text(encoding="utf-8")
+        self.recipes = {
+            name: body
+            for name, body in re.findall(
+                r"^(run-dev|run-prod):[^\n]*\n((?:\t[^\n]*\n)+)",
+                self.makefile,
+                re.MULTILINE,
+            )
+        }
+
+    def test_both_run_targets_exist(self):
+        self.assertEqual(set(self.recipes), {"run-dev", "run-prod"})
+
+    def test_the_container_is_told_which_hosts_are_trusted(self):
+        for name, body in self.recipes.items():
+            with self.subTest(target=name):
+                self.assertIn("TRUSTED_HOSTS", body)
+
+    def test_the_container_is_told_which_port_to_bind(self):
+        for name, body in self.recipes.items():
+            with self.subTest(target=name):
+                self.assertIn('-e PORT="$$PORT"', body)
+
+    def test_the_whole_env_file_is_not_handed_to_the_web_container(self):
+        for name, body in self.recipes.items():
+            with self.subTest(target=name):
+                self.assertNotIn("--env-file", body)
+
+
+class TestLintCoverage(unittest.TestCase):
+    def setUp(self):
+        self.makefile = (REPO_ROOT / "Makefile").read_text(encoding="utf-8")
+
+    def test_the_lint_target_runs_every_linter(self):
+        prerequisites = re.search(r"^lint: (.+)$", self.makefile, re.MULTILINE).group(1)
+
+        self.assertGreaterEqual(
+            set(prerequisites.split()),
+            {"lint-actions", "lint-python", "lint-yaml", "lint-js", "lint-shell"},
+        )
+
+    def test_every_linter_has_a_ci_job(self):
+        workflow = yaml.safe_load(
+            (REPO_ROOT / ".github" / "workflows" / "lint.yml").read_text(
+                encoding="utf-8"
+            )
+        )
+
+        self.assertGreaterEqual(
+            set(workflow["jobs"]),
+            {"lint-actions", "lint-python", "lint-yaml", "lint-js", "lint-shell"},
+        )
+
+    def test_the_javascript_linter_is_declared(self):
+        package = json.loads(
+            (REPO_ROOT / "app" / "package.json").read_text(encoding="utf-8")
+        )
+
+        self.assertGreaterEqual(
+            set(package["devDependencies"]), {"eslint", "@eslint/js", "globals"}
+        )
+
+    def test_the_documented_environment_keys_exist(self):
+        example = (REPO_ROOT / "env.example").read_text(encoding="utf-8")
+
+        for key in ("PORT", "IMAGE_NAME", "TRUSTED_HOSTS", "LIBRETRANSLATE_URL"):
+            with self.subTest(key=key):
+                self.assertRegex(example, rf"(?m)^{key}=")
+
+
+class TestEndToEndRunner(unittest.TestCase):
+    def setUp(self):
+        self.script = (REPO_ROOT / "scripts" / "run-e2e.sh").read_text(encoding="utf-8")
+
+    def test_a_foreign_listener_stops_the_run(self):
+        self.assertIn("already serves port", self.script)
+
+    def test_cypress_is_pinned_to_the_origin_flask_binds(self):
+        self.assertIn("CYPRESS_baseUrl", self.script)
+        self.assertIn("127.0.0.1", self.script)
+
+    def test_the_electron_node_flag_is_dropped(self):
+        self.assertIn("env -u ELECTRON_RUN_AS_NODE", self.script)
+
+    def test_every_probe_bypasses_a_proxy_and_is_bounded(self):
+        probes = [line for line in self.script.splitlines() if "curl " in line]
+
+        self.assertTrue(probes)
+        for probe in probes:
+            with self.subTest(probe=probe.strip()):
+                self.assertIn("--noproxy", probe)
+                self.assertIn("--max-time", probe)
+
+
+class TestVendoredAssets(unittest.TestCase):
+    def test_the_right_to_left_stylesheet_is_vendored(self):
+        script = (REPO_ROOT / "app" / "scripts" / "copy-vendor.js").read_text(
+            encoding="utf-8"
+        )
+
+        self.assertEqual(
+            script.count("bootstrap.rtl.min.css"),
+            2,
+            "the RTL stylesheet needs both a source and a destination path",
+        )
+
+
+class TestPackagedCatalogs(unittest.TestCase):
+    def test_the_interface_catalogs_are_declared_as_package_data(self):
+        with (REPO_ROOT / "pyproject.toml").open("rb") as handle:
+            pyproject = tomllib.load(handle)
+
+        package_data = pyproject["tool"]["setuptools"]["package-data"]["app"]
+
+        self.assertIn("i18n/ui/*.yaml", package_data)
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/unit/test_i18n.py

@@ -0,0 +1,230 @@
+import re
+import shutil
+import tempfile
+import unittest
+from pathlib import Path
+
+import yaml
+
+from app.utils import i18n
+
+
+class TestNegotiate(unittest.TestCase):
+    def test_regional_tag_beats_a_lower_ranked_exact_match(self):
+        self.assertEqual(i18n.negotiate([("de-DE", 1.0), ("en", 0.8)]), "de")
+
+    def test_an_uppercase_tag_is_accepted(self):
+        self.assertEqual(i18n.negotiate([("DE-DE", 1.0)]), "de")
+
+    def test_underscore_separated_tag_is_accepted(self):
+        self.assertEqual(i18n.negotiate([("pt_BR", 1.0)]), "pt")
+
+    def test_highest_quality_supported_tag_wins(self):
+        self.assertEqual(
+            i18n.negotiate([("xx", 1.0), ("fr", 0.9), ("es", 0.5)]),
+            "fr",
+        )
+
+    def test_unsupported_tags_fall_back_to_the_default(self):
+        self.assertEqual(i18n.negotiate([("xx", 1.0)]), "en")
+        self.assertEqual(i18n.negotiate([], default="de"), "de")
+
+    def test_a_refused_language_is_not_selected(self):
+        self.assertEqual(i18n.negotiate([("de", 0.0)]), "en")
+
+    def test_the_first_of_two_equal_tags_wins(self):
+        self.assertEqual(i18n.negotiate([("fr", 0.9), ("es", 0.9)]), "fr")
+
+
+class TestDirection(unittest.TestCase):
+    def test_every_right_to_left_language_is_marked(self):
+        for code in ("ar", "dv", "fa", "he", "ks", "ps", "sd", "ug", "ur", "yi"):
+            with self.subTest(code=code):
+                self.assertEqual(i18n.direction(code), "rtl")
+
+    def test_other_languages_are_left_to_right(self):
+        for code in set(i18n.LANGUAGES) - set(i18n.RTL_LANGUAGES):
+            with self.subTest(code=code):
+                self.assertEqual(i18n.direction(code), "ltr")
+
+
+class TestTranslateTree(unittest.TestCase):
+    def setUp(self):
+        self.addCleanup(i18n._catalogs.clear)
+        i18n._catalogs["xx"] = {"A card": "Eine Karte", "Pictures": "Bilder"}
+
+    def test_only_translatable_keys_are_replaced(self):
+        tree = {
+            "cards": [
+                {
+                    "title": "Pictures",
+                    "text": "A card",
+                    "url": "A card",
+                    "icon": {"class": "Pictures"},
+                }
+            ]
+        }
+
+        translated = i18n.translate_tree(tree, "xx")
+
+        card = translated["cards"][0]
+        self.assertEqual(card["title"], "Bilder")
+        self.assertEqual(card["text"], "Eine Karte")
+        self.assertEqual(card["url"], "A card")
+        self.assertEqual(card["icon"]["class"], "Pictures")
+
+    def test_strings_inside_a_list_are_translated(self):
+        translated = i18n.translate_tree({"text": ["A card", "Pictures"]}, "xx")
+
+        self.assertEqual(translated["text"], ["Eine Karte", "Bilder"])
+
+    def test_unknown_strings_keep_their_source_value(self):
+        translated = i18n.translate_tree({"description": "Untranslated"}, "xx")
+
+        self.assertEqual(translated["description"], "Untranslated")
+
+    def test_source_tree_is_left_untouched(self):
+        tree = {"name": "Pictures"}
+
+        i18n.translate_tree(tree, "xx")
+
+        self.assertEqual(tree["name"], "Pictures")
+
+    def test_non_string_leaves_survive(self):
+        tree = {"name": 1, "text": None, "info": True}
+
+        self.assertEqual(i18n.translate_tree(tree, "xx"), tree)
+
+
+class TestReadCatalog(unittest.TestCase):
+    def setUp(self):
+        self.directory = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.directory, True)
+        self.path = self.directory / "de.yaml"
+
+    def _read(self, text):
+        self.path.write_text(text, encoding="utf-8")
+        with self.assertLogs(level="WARNING"):
+            return i18n.read_catalog(self.path)
+
+    def test_absent_file_is_an_empty_catalog(self):
+        self.assertEqual(i18n.read_catalog(self.directory / "missing.yaml"), {})
+
+    def test_unparsable_yaml_is_an_empty_catalog(self):
+        self.assertEqual(self._read("Close: [unclosed"), {})
+
+    def test_tab_indentation_is_an_empty_catalog(self):
+        self.assertEqual(self._read("a:\n\tb: 1"), {})
+
+    def test_non_mapping_document_is_an_empty_catalog(self):
+        self.assertEqual(self._read("- a\n- b"), {})
+
+    def test_a_non_utf8_catalog_is_an_empty_catalog(self):
+        self.path.write_bytes(b"\xffClose: Schliessen\n")
+
+        with self.assertLogs(level="WARNING"):
+            self.assertEqual(i18n.read_catalog(self.path), {})
+
+    def test_a_directory_at_the_catalog_path_is_an_empty_catalog(self):
+        (self.directory / "sub.yaml").mkdir()
+
+        with self.assertLogs(level="WARNING"):
+            self.assertEqual(i18n.read_catalog(self.directory / "sub.yaml"), {})
+
+    def test_non_string_entries_are_dropped(self):
+        self.path.write_text(
+            "Close: 42\nOpen:\nCopy: yes\n123: Zahl\nyes: Ja\nImprint: Impressum\n",
+            encoding="utf-8",
+        )
+
+        self.assertEqual(i18n.read_catalog(self.path), {"Imprint": "Impressum"})
+
+    def test_a_missing_catalog_is_silent(self):
+        with self.assertNoLogs(level="WARNING"):
+            i18n.read_catalog(self.directory / "absent.yaml")
+
+
+class TestCatalogMerge(unittest.TestCase):
+    def setUp(self):
+        directory = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, directory, True)
+        (directory / "ui").mkdir()
+        (directory / "content").mkdir()
+
+        self.addCleanup(setattr, i18n, "UI_DIR", i18n.UI_DIR)
+        self.addCleanup(setattr, i18n, "CONTENT_DIR", i18n.CONTENT_DIR)
+        self.addCleanup(i18n.clear_catalogs)
+        i18n.UI_DIR = directory / "ui"
+        i18n.CONTENT_DIR = directory / "content"
+        i18n.clear_catalogs()
+
+    def test_the_content_catalog_overrides_the_shipped_interface_string(self):
+        (i18n.UI_DIR / "de.yaml").write_text(
+            "Close: Schliessen\nImprint: Impressum\n", encoding="utf-8"
+        )
+        (i18n.CONTENT_DIR / "de.yaml").write_text("Close: Zumachen\n", encoding="utf-8")
+
+        self.assertEqual(
+            i18n.catalog("de"), {"Close": "Zumachen", "Imprint": "Impressum"}
+        )
+
+
+class TestShippedCatalogs(unittest.TestCase):
+    def test_every_iso_639_1_language_is_offered(self):
+        self.assertEqual(len(i18n.LANGUAGES), 184)
+        self.assertEqual(i18n.SOURCE_LANGUAGE, next(iter(i18n.LANGUAGES)))
+        self.assertGreaterEqual(
+            set(i18n.LANGUAGES), {"en", "de", "zh", "ar", "he", "vo", "za", "cu"}
+        )
+
+    def test_every_display_name_is_filled_in(self):
+        blank = [code for code, name in i18n.LANGUAGES.items() if not name.strip()]
+
+        self.assertFalse(blank, f"No display name for: {blank}")
+
+    def test_every_code_is_usable_in_the_route_converter(self):
+        offenders = [
+            code for code in i18n.LANGUAGES if not re.fullmatch(r"[a-z]+", code)
+        ]
+
+        self.assertFalse(offenders, f"Not usable in the route converter: {offenders}")
+
+    def test_every_shipped_catalog_names_a_known_language(self):
+        stray = [
+            path.stem
+            for path in i18n.UI_DIR.glob("*.yaml")
+            if path.stem not in i18n.LANGUAGES or path.stem == i18n.SOURCE_LANGUAGE
+        ]
+
+        self.assertFalse(stray, f"Catalogue for an unknown language: {stray}")
+
+    def test_ui_catalogs_cover_exactly_the_interface_strings(self):
+        expected = set(i18n.UI_STRINGS)
+        mismatched = {}
+
+        for path in sorted(i18n.UI_DIR.glob("*.yaml")):
+            code = path.stem
+            entries = yaml.safe_load(path.read_text(encoding="utf-8"))
+            if set(entries) != expected:
+                mismatched[code] = {
+                    "missing": sorted(expected - set(entries)),
+                    "unexpected": sorted(set(entries) - expected),
+                }
+
+        self.assertFalse(mismatched, f"UI catalogues out of sync: {mismatched}")
+
+    def test_ui_strings_of_the_source_language_are_the_source(self):
+        self.assertEqual(
+            i18n.ui_strings("en"),
+            {source: source for source in i18n.UI_STRINGS},
+        )
+
+    def test_ui_strings_are_translated_for_a_shipped_language(self):
+        strings = i18n.ui_strings("de")
+
+        self.assertEqual(strings["Close"], "Schließen")
+        self.assertEqual(strings["Imprint"], "Impressum")
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/unit/test_i18n_sync.py

@@ -0,0 +1,454 @@
+import shutil
+import tempfile
+import unittest
+from pathlib import Path
+from unittest.mock import Mock, patch
+
+import yaml
+
+from app.utils import i18n
+from utils import i18n_sync
+
+
+class TestCollectSources(unittest.TestCase):
+    def test_prose_keys_are_collected_from_nested_structures(self):
+        config = {
+            "cards": [
+                {"title": "Agile Coach", "text": "I lead transformations."},
+                {"text": "Another card."},
+            ],
+            "navigation": {
+                "header": {
+                    "children": [
+                        {"name": "Apps", "description": "Application menu"},
+                    ]
+                }
+            },
+            "platform": {"titel": "Someone", "subtitel": "A tagline"},
+        }
+
+        self.assertEqual(
+            i18n_sync.collect_sources(config),
+            {
+                "Agile Coach",
+                "I lead transformations.",
+                "Another card.",
+                "Apps",
+                "Application menu",
+                "A tagline",
+            },
+        )
+
+    def test_labels_are_collected_too(self):
+        config = {"name": "Pictures", "title": "Agile Coach"}
+
+        self.assertEqual(i18n_sync.collect_sources(config), {"Pictures", "Agile Coach"})
+
+    def test_structural_keys_are_left_alone(self):
+        config = {
+            "url": "https://example.test",
+            "link_text": "www.example.test",
+            "identifier": "@someone@example.test",
+            "icon": {"class": "fa-solid fa-users"},
+        }
+
+        self.assertEqual(i18n_sync.collect_sources(config), set())
+
+    def test_blank_values_are_ignored(self):
+        self.assertEqual(i18n_sync.collect_sources({"text": "   "}), set())
+
+    def test_a_list_of_prose_is_collected(self):
+        self.assertEqual(
+            i18n_sync.collect_sources({"text": ["one", "two"]}), {"one", "two"}
+        )
+
+
+class TestTranslate(unittest.TestCase):
+    def _session(self, ok, payload=None, status=200, text=""):
+        response = Mock(ok=ok, status_code=status, text=text)
+        response.json.return_value = payload or {}
+        return Mock(post=Mock(return_value=response))
+
+    def test_successful_response_returns_the_translation(self):
+        session = self._session(True, {"translatedText": "Hallo"})
+
+        result = i18n_sync.translate(session, "http://lt", "", "Hello", "de")
+
+        self.assertEqual(result, "Hallo")
+
+    def test_api_key_is_only_sent_when_configured(self):
+        session = self._session(True, {"translatedText": "Hallo"})
+
+        i18n_sync.translate(session, "http://lt", "secret", "Hello", "de")
+
+        self.assertEqual(session.post.call_args.kwargs["data"]["api_key"], "secret")
+
+    def test_no_api_key_is_sent_when_none_is_configured(self):
+        session = self._session(True, {"translatedText": "Hallo"})
+
+        i18n_sync.translate(session, "http://lt", "", "Hello", "de")
+
+        self.assertNotIn("api_key", session.post.call_args.kwargs["data"])
+
+    def test_failed_response_returns_none(self):
+        session = self._session(
+            False, {"translatedText": "SHOULD NOT BE USED"}, status=403, text="denied"
+        )
+
+        self.assertIsNone(i18n_sync.translate(session, "http://lt", "", "Hi", "de"))
+
+    def test_the_request_asks_for_plain_text_from_the_source_language(self):
+        session = self._session(True, {"translatedText": "Hallo"})
+
+        i18n_sync.translate(session, "http://lt", "", "Hello", "de")
+
+        data = session.post.call_args.kwargs["data"]
+        self.assertEqual(data["format"], "text")
+        self.assertEqual(data["source"], i18n.SOURCE_LANGUAGE)
+        self.assertEqual(data["target"], "de")
+
+    def test_the_request_is_bounded_by_a_timeout(self):
+        session = self._session(True, {"translatedText": "Hallo"})
+
+        i18n_sync.translate(session, "http://lt", "", "Hello", "de")
+
+        timeout = session.post.call_args.kwargs["timeout"]
+        self.assertIsInstance(timeout, (int, float))
+        self.assertGreater(timeout, 0)
+
+    def test_an_empty_translation_is_refused(self):
+        session = self._session(True, {"translatedText": ""})
+
+        self.assertIsNone(i18n_sync.translate(session, "http://lt", "", "Hi", "de"))
+
+    def test_a_non_json_success_response_returns_none(self):
+        response = Mock(ok=True)
+        response.json.side_effect = ValueError("no json")
+        session = Mock(post=Mock(return_value=response))
+
+        self.assertIsNone(i18n_sync.translate(session, "http://lt", "", "Hi", "de"))
+
+    def test_a_transport_failure_returns_none(self):
+        session = Mock(
+            post=Mock(side_effect=i18n_sync.requests.ConnectionError("reset"))
+        )
+
+        self.assertIsNone(i18n_sync.translate(session, "http://lt", "", "Hi", "de"))
+
+    def test_a_non_string_translation_is_refused(self):
+        session = self._session(True, {"translatedText": ["Hallo", "Welt"]})
+
+        self.assertIsNone(i18n_sync.translate(session, "http://lt", "", "Hi", "de"))
+
+
+class TestSupportedLanguages(unittest.TestCase):
+    def _session(self, payload=None, side_effect=None):
+        response = Mock(raise_for_status=Mock())
+        if side_effect is not None:
+            response.json.side_effect = side_effect
+        else:
+            response.json.return_value = payload
+        return Mock(get=Mock(return_value=response))
+
+    def test_the_offered_codes_are_returned(self):
+        session = self._session([{"code": "de"}, {"code": "fr"}])
+
+        self.assertEqual(
+            i18n_sync.supported_languages("http://lt", session), {"de", "fr"}
+        )
+
+    def test_an_unreachable_instance_is_reported(self):
+        session = Mock(get=Mock(side_effect=i18n_sync.requests.ConnectTimeout("slow")))
+
+        with self.assertRaises(i18n_sync.BackendError):
+            i18n_sync.supported_languages("http://lt", session)
+
+    def test_an_error_status_is_reported(self):
+        response = Mock()
+        response.raise_for_status.side_effect = i18n_sync.requests.HTTPError("503")
+        response.json.return_value = [{"code": "de"}]
+        session = Mock(get=Mock(return_value=response))
+
+        with self.assertRaises(i18n_sync.BackendError):
+            i18n_sync.supported_languages("http://lt", session)
+
+    def test_a_non_json_listing_is_reported(self):
+        session = self._session(side_effect=ValueError("no json"))
+
+        with self.assertRaises(i18n_sync.BackendError):
+            i18n_sync.supported_languages("http://lt", session)
+
+    def test_json_of_the_wrong_shape_is_reported(self):
+        for payload in ({"error": "Slow down"}, ["de", "fr"], None, [{"name": "de"}]):
+            with self.subTest(payload=payload):
+                with self.assertRaises(i18n_sync.BackendError):
+                    i18n_sync.supported_languages("http://lt", self._session(payload))
+
+
+class TestReadKeep(unittest.TestCase):
+    def setUp(self):
+        self.directory = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.directory, True)
+        self.path = self.directory / "keep.txt"
+
+    def test_a_missing_file_keeps_nothing(self):
+        self.assertEqual(i18n_sync.read_keep(self.path), [])
+
+    def test_comments_and_blank_lines_are_ignored(self):
+        self.path.write_text(
+            "# brands\n\nMastodon\n  Bluesky  \n\n# more\nNextcloud\n",
+            encoding="utf-8",
+        )
+
+        self.assertEqual(
+            i18n_sync.read_keep(self.path), ["Mastodon", "Bluesky", "Nextcloud"]
+        )
+
+    def test_the_shipped_list_names_the_brands_of_the_sample_configuration(self):
+        shipped = i18n_sync.read_keep(i18n_sync.DEFAULT_KEEP_PATH)
+
+        self.assertGreaterEqual(set(shipped), {"Mastodon", "Nextcloud", "Matrix"})
+        self.assertNotIn("Pictures", shipped)
+        self.assertNotIn("Imprint", shipped)
+
+
+class TestCommandLine(unittest.TestCase):
+    def setUp(self):
+        self.directory = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.directory, True)
+        self.keep_file = self.directory / "keep.txt"
+        self.config = self.directory / "config.yaml"
+        self.config.write_text("cards: []\n", encoding="utf-8")
+
+    def _main(self, *extra):
+        with patch.object(i18n_sync, "sync") as sync:
+            i18n_sync.main(
+                [
+                    "--url",
+                    "http://lt/",
+                    "--config",
+                    str(self.config),
+                    "--keep-file",
+                    str(self.keep_file),
+                    *extra,
+                ]
+            )
+        return sync.call_args
+
+    def test_the_keep_file_reaches_the_sync(self):
+        self.keep_file.write_text("# brands\nMastodon\nNextcloud\n", encoding="utf-8")
+
+        keep = self._main().args[5]
+
+        self.assertEqual(list(keep), ["Mastodon", "Nextcloud"])
+
+    def test_the_command_line_adds_to_the_keep_file(self):
+        self.keep_file.write_text("Mastodon\n", encoding="utf-8")
+
+        keep = self._main("--keep", "Taiga").args[5]
+
+        self.assertEqual(list(keep), ["Mastodon", "Taiga"])
+
+    def test_a_trailing_slash_is_stripped_from_the_url(self):
+        self.assertEqual(self._main().args[0], "http://lt")
+
+    def test_a_backend_failure_is_reported_as_an_exit_code(self):
+        with patch.object(i18n_sync, "sync", side_effect=i18n_sync.BackendError("no")):
+            self.assertEqual(
+                i18n_sync.main(["--url", "http://lt", "--config", str(self.config)]), 1
+            )
+
+
+class TestLoadExisting(unittest.TestCase):
+    def setUp(self):
+        self.directory = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.directory, True)
+        self.path = self.directory / "de.yaml"
+
+    def test_a_missing_file_is_an_empty_catalog(self):
+        self.assertEqual(i18n_sync.load_existing(self.path), {})
+
+    def test_a_readable_catalog_is_returned(self):
+        self.path.write_text("Hi: Hallo\n", encoding="utf-8")
+
+        self.assertEqual(i18n_sync.load_existing(self.path), {"Hi": "Hallo"})
+
+    def test_an_unparsable_catalog_is_refused(self):
+        self.path.write_text('Hi: "unterminated\n', encoding="utf-8")
+
+        self.assertIsNone(i18n_sync.load_existing(self.path))
+
+    def test_a_non_mapping_catalog_is_refused(self):
+        self.path.write_text("- a\n- b\n", encoding="utf-8")
+
+        self.assertIsNone(i18n_sync.load_existing(self.path))
+
+    def test_a_non_utf8_catalog_is_refused(self):
+        self.path.write_bytes(b"\xffHi: Hallo\n")
+
+        self.assertIsNone(i18n_sync.load_existing(self.path))
+
+    def test_a_directory_at_the_catalog_path_is_refused(self):
+        (self.directory / "sub.yaml").mkdir()
+
+        self.assertIsNone(i18n_sync.load_existing(self.directory / "sub.yaml"))
+
+    def test_an_empty_or_comment_only_catalog_is_an_empty_catalog(self):
+        for text in ("", "\n\n", "# only a comment\n"):
+            with self.subTest(text=text):
+                self.path.write_text(text, encoding="utf-8")
+                self.assertEqual(i18n_sync.load_existing(self.path), {})
+
+
+class TestWriteCatalog(unittest.TestCase):
+    def setUp(self):
+        self.directory = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.directory, True)
+        self.path = self.directory / "de.yaml"
+
+    def test_the_catalog_is_written_and_no_scratch_file_is_left(self):
+        i18n_sync.write_catalog(self.path, {"Hi": "Hallo", "Umlaut": "Grüße"})
+
+        text = self.path.read_text(encoding="utf-8")
+        self.assertEqual(yaml.safe_load(text), {"Hi": "Hallo", "Umlaut": "Grüße"})
+        self.assertIn("Grüße", text)
+        self.assertEqual([p.name for p in self.directory.iterdir()], ["de.yaml"])
+
+    def test_a_write_that_dies_halfway_leaves_the_previous_catalog_intact(self):
+        self.path.write_text("Hi: HAND-EDITED\n", encoding="utf-8")
+        complete = Path.write_text
+
+        def dies_halfway(target, data, *args, **kwargs):
+            complete(target, data[: len(data) // 2], *args, **kwargs)
+            raise OSError("no space left on device")
+
+        with patch.object(Path, "write_text", dies_halfway):
+            with self.assertRaises(OSError):
+                i18n_sync.write_catalog(self.path, {"Hi": "machine", "Zebra": "Zebra"})
+
+        self.assertEqual(self.path.read_text(encoding="utf-8"), "Hi: HAND-EDITED\n")
+
+
+class TestSync(unittest.TestCase):
+    def setUp(self):
+        self.directory = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.directory, True)
+        self.addCleanup(setattr, i18n, "CONTENT_DIR", i18n.CONTENT_DIR)
+        i18n.CONTENT_DIR = self.directory
+        self.path = self.directory / "de.yaml"
+
+    def _run(self, sources, offered=("de",), translated="UEBERSETZT", keep=()):
+        listing = Mock(raise_for_status=Mock())
+        listing.json.return_value = [{"code": code} for code in offered]
+        answer = Mock(ok=True)
+        answer.json.return_value = {"translatedText": translated}
+        session = Mock(get=Mock(return_value=listing), post=Mock(return_value=answer))
+
+        with patch.object(i18n_sync.requests, "Session", return_value=session):
+            i18n_sync.sync("http://lt", "", set(sources), ["de"], self.directory, keep)
+        return session
+
+    def test_missing_entries_are_written(self):
+        self._run(["Hello"])
+
+        self.assertEqual(
+            yaml.safe_load(self.path.read_text(encoding="utf-8")),
+            {"Hello": "UEBERSETZT"},
+        )
+
+    def test_existing_entries_are_never_overwritten(self):
+        self.path.write_text("Hello: HAND-EDITED\n", encoding="utf-8")
+
+        session = self._run(["Hello", "World"])
+
+        catalog = yaml.safe_load(self.path.read_text(encoding="utf-8"))
+        self.assertEqual(catalog["Hello"], "HAND-EDITED")
+        self.assertEqual(session.post.call_count, 1)
+
+    def test_an_unparsable_catalog_is_left_alone(self):
+        broken = 'Hello: "unterminated\n'
+        self.path.write_text(broken, encoding="utf-8")
+
+        session = self._run(["Hello"])
+
+        self.assertEqual(self.path.read_text(encoding="utf-8"), broken)
+        session.post.assert_not_called()
+
+    def test_a_language_the_instance_does_not_offer_is_skipped(self):
+        session = self._run(["Hello"], offered=("fr",))
+
+        self.assertFalse(self.path.exists())
+        session.post.assert_not_called()
+
+    def test_a_protected_string_is_stored_as_itself(self):
+        session = self._run(["Mastodon", "Hello"], keep=["Mastodon"])
+
+        catalog = yaml.safe_load(self.path.read_text(encoding="utf-8"))
+        self.assertEqual(catalog["Mastodon"], "Mastodon")
+        self.assertEqual(session.post.call_count, 1)
+
+    def test_a_protected_string_never_overwrites_an_existing_entry(self):
+        self.path.write_text("Mastodon: HAND-EDITED\n", encoding="utf-8")
+
+        self._run(["Mastodon", "Hello"], keep=["Mastodon"])
+
+        catalog = yaml.safe_load(self.path.read_text(encoding="utf-8"))
+        self.assertEqual(catalog["Mastodon"], "HAND-EDITED")
+        self.assertEqual(catalog["Hello"], "UEBERSETZT")
+
+    def test_an_empty_translation_is_not_stored(self):
+        self._run(["Hello"], translated="")
+
+        self.assertFalse(self.path.exists())
+
+    def test_a_run_that_translated_nothing_leaves_the_file_untouched(self):
+        original = "# Reviewed by a native speaker, keep the order.\nZebra: Zebra\n"
+        self.path.write_text(original, encoding="utf-8")
+
+        self._run(["Hello"], translated=None)
+
+        self.assertEqual(self.path.read_text(encoding="utf-8"), original)
+
+    def test_a_string_the_shipped_catalog_covers_is_not_requested(self):
+        ui = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, ui, True)
+        self.addCleanup(setattr, i18n, "UI_DIR", i18n.UI_DIR)
+        i18n.UI_DIR = ui
+        (ui / "de.yaml").write_text("Close: Schliessen\n", encoding="utf-8")
+
+        session = self._run(["Close", "Hello"])
+
+        self.assertEqual(session.post.call_count, 1)
+        self.assertNotIn("Close", yaml.safe_load(self.path.read_text(encoding="utf-8")))
+
+    def test_a_language_that_cannot_be_written_does_not_stop_the_others(self):
+        listing = Mock(raise_for_status=Mock())
+        listing.json.return_value = [{"code": "de"}, {"code": "fr"}]
+        answer = Mock(ok=True)
+        answer.json.return_value = {"translatedText": "UEBERSETZT"}
+        session = Mock(get=Mock(return_value=listing), post=Mock(return_value=answer))
+        original = i18n_sync.write_catalog
+
+        def fails_for_german(path, catalog):
+            if path.name == "de.yaml":
+                raise OSError("read-only")
+            original(path, catalog)
+
+        with patch.object(i18n_sync, "write_catalog", fails_for_german):
+            with patch.object(i18n_sync.requests, "Session", return_value=session):
+                i18n_sync.sync("http://lt", "", {"Hello"}, ["de", "fr"], self.directory)
+
+        self.assertTrue((self.directory / "fr.yaml").is_file())
+
+    def test_the_catalog_directory_is_created(self):
+        nested = self.directory / "content"
+        self.directory = nested
+        self.path = nested / "de.yaml"
+
+        self._run(["Hello"])
+
+        self.assertTrue(self.path.is_file())
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/unit/test_navigation_template.py

@@ -2,7 +2,7 @@ import unittest
 from html.parser import HTMLParser
 from pathlib import Path
 
-from jinja2 import Environment, FileSystemLoader, select_autoescape
+from jinja2 import Environment, FileSystemLoader
 
 
 class AnchorCollector(HTMLParser):
@@ -20,10 +20,12 @@ class TestNavigationTemplate(unittest.TestCase):
         template_dir = Path(__file__).resolve().parents[2] / "app" / "templates"
         environment = Environment(
             loader=FileSystemLoader(template_dir),
-            autoescape=select_autoescape(),
+            autoescape=True,
         )
-        environment.globals["url_for"] = lambda _endpoint, filename: (
-            f"/static/{filename}"
+        environment.globals["url_for"] = lambda endpoint, **kwargs: (
+            f"/static/{kwargs['filename']}"
+            if endpoint == "static"
+            else f"/{kwargs['lang']}/"
         )
         environment.globals["asset_src"] = lambda asset: (
             (asset or {}).get("external_url")
@@ -36,6 +38,9 @@ class TestNavigationTemplate(unittest.TestCase):
 
         rendered = environment.get_template("moduls/navigation.html.j2").render(
             menu_type="header",
+            lang="en",
+            languages={"en": "English", "de": "Deutsch"},
+            t=lambda source: source,
             platform={
                 "titel": "Portfolio",
                 "logo": {"cache": "logo.png"},
@@ -70,8 +75,15 @@ class TestNavigationTemplate(unittest.TestCase):
             and "dropdown-toggle" in anchor.get("class", "")
         ]
 
-        self.assertEqual(len(dropdown_toggles), 1)
-        self.assertEqual(dropdown_toggles[0].get("data-bs-toggle"), "dropdown")
+        self.assertEqual(len(dropdown_toggles), 2)
+        for toggle in dropdown_toggles:
+            self.assertEqual(toggle.get("data-bs-toggle"), "dropdown")
+
+        language_links = [
+            anchor for anchor in parser.anchors if anchor.get("hreflang") == "de"
+        ]
+        self.assertEqual(len(language_links), 1)
+        self.assertEqual(language_links[0]["href"], "/de/")
 
 
 if __name__ == "__main__":

utils/generate_languages.py

@@ -0,0 +1,83 @@
+#!/usr/bin/env python3
+"""Regenerate app/utils/languages.py from ISO 639-1 and CLDR.
+
+Every ISO 639-1 alpha-2 code becomes an entry. The display name is the CLDR
+endonym where CLDR knows the language, and the English ISO name for the
+remainder. Writing direction comes from CLDR's character order.
+
+Needs the dev extra: pip install -e ".[dev]"
+"""
+
+import subprocess
+import sys
+from pathlib import Path
+
+import pycountry
+from babel import Locale
+from babel.localedata import locale_identifiers
+
+TARGET = Path(__file__).resolve().parents[1] / "app" / "utils" / "languages.py"
+
+HEADER = '''"""ISO 639-1 languages, their display names and writing direction.
+
+Generated by utils/generate_languages.py — edit that script, not this file.
+
+Display names are CLDR endonyms where CLDR covers the language and the English
+ISO 639-1 name for the {fallback} codes it does not.
+"""
+
+LANGUAGES = {{
+'''
+
+FOOTER = """}}
+
+RTL_LANGUAGES = frozenset({rtl})
+"""
+
+
+def collect():
+    """Return (code -> display name) and the set of right-to-left codes."""
+    cldr = {code for code in locale_identifiers() if len(code) == 2 and code.isalpha()}
+    names, rtl, fallbacks = {}, set(), 0
+
+    for language in sorted(
+        pycountry.languages, key=lambda item: getattr(item, "alpha_2", "")
+    ):
+        code = getattr(language, "alpha_2", None)
+        if not code:
+            continue
+        if code in cldr:
+            locale = Locale.parse(code)
+            names[code] = locale.get_display_name(code)
+            if locale.character_order == "right-to-left":
+                rtl.add(code)
+        else:
+            names[code] = language.name
+            fallbacks += 1
+
+    return names, rtl, fallbacks
+
+
+def render(names, rtl, fallbacks):
+    """Return the module source, English first and the rest by code."""
+    ordered = ["en"] + [code for code in sorted(names) if code != "en"]
+    body = "".join(f'    "{code}": "{names[code]}",\n' for code in ordered)
+    listed = ", ".join(f'"{code}"' for code in sorted(rtl))
+    return (
+        HEADER.format(fallback=fallbacks) + body + FOOTER.format(rtl="{" + listed + "}")
+    )
+
+
+def main():
+    names, rtl, fallbacks = collect()
+    TARGET.write_text(render(names, rtl, fallbacks), encoding="utf-8")
+    subprocess.run([sys.executable, "-m", "ruff", "format", str(TARGET)], check=False)
+    print(
+        f"{len(names)} languages, {len(rtl)} right-to-left, "
+        f"{fallbacks} without a CLDR endonym"
+    )
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

utils/i18n_sync.py

@@ -0,0 +1,306 @@
+#!/usr/bin/env python3
+"""Fill missing content translations from a LibreTranslate instance.
+
+Reads the prose strings out of the live ``app/config.yaml`` and writes one
+catalogue per language to ``app/i18n/content/``. Entries that already exist are
+never overwritten, so a hand-corrected translation survives every later run.
+"""
+
+import argparse
+import os
+import sys
+from pathlib import Path
+
+import requests
+import yaml
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT))
+
+from app.utils import i18n  # noqa: E402
+
+DEFAULT_CONFIG_PATH = REPO_ROOT / "app" / "config.yaml"
+DEFAULT_KEEP_PATH = i18n.I18N_DIR / "keep.txt"
+REQUEST_TIMEOUT = 30
+
+
+def collect_sources(node, key=None, found=None):
+    """Return every prose string in ``node`` that a backend may translate.
+
+    Args:
+        node: the raw configuration tree, or any subtree of it.
+        key: the mapping key ``node`` was reached through.
+        found: accumulator, for recursion.
+    """
+    found = set() if found is None else found
+    if isinstance(node, dict):
+        for name, value in node.items():
+            collect_sources(value, name, found)
+    elif isinstance(node, list):
+        for item in node:
+            collect_sources(item, key, found)
+    elif isinstance(node, str) and key in i18n.TRANSLATABLE_KEYS and node.strip():
+        found.add(node)
+    return found
+
+
+def read_keep(path):
+    """Return the strings listed in ``path``, ignoring blanks and comments.
+
+    Args:
+        path: a text file with one string per line, or a path that does not exist.
+    """
+    if not path.is_file():
+        return []
+    lines = path.read_text(encoding="utf-8").splitlines()
+    return [
+        stripped
+        for stripped in (line.strip() for line in lines)
+        if stripped and not stripped.startswith("#")
+    ]
+
+
+class BackendError(Exception):
+    """The translation backend answered in a way the run cannot continue from."""
+
+
+def supported_languages(url, session):
+    """Return the language codes the LibreTranslate instance at ``url`` offers."""
+    try:
+        response = session.get(f"{url}/languages", timeout=REQUEST_TIMEOUT)
+        response.raise_for_status()
+        offered = response.json()
+    except requests.RequestException as error:
+        raise BackendError(f"{url} is not reachable: {error}")
+    except ValueError:
+        raise BackendError(f"{url}/languages did not answer JSON")
+
+    if not isinstance(offered, list):
+        raise BackendError(
+            f"{url}/languages answered {type(offered).__name__}, not a list"
+        )
+    codes = {
+        entry["code"]
+        for entry in offered
+        if isinstance(entry, dict) and isinstance(entry.get("code"), str)
+    }
+    if not codes:
+        raise BackendError(f"{url}/languages listed no usable language codes")
+    return codes
+
+
+def load_existing(path):
+    """Return the catalogue at ``path``, or None when it must not be rewritten.
+
+    Distinct from ``i18n.read_catalog``, which degrades an unreadable catalogue
+    to English at render time. Here the same file means the hand-written entries
+    are unknown, and writing would replace them with a fresh machine pass.
+    """
+    if not path.exists():
+        return {}
+    try:
+        loaded = yaml.safe_load(path.read_text(encoding="utf-8"))
+    except (OSError, UnicodeDecodeError, yaml.YAMLError) as error:
+        print(f"  ! {path.name}: {type(error).__name__}, refusing to overwrite it")
+        return None
+    if loaded is None:
+        return {}
+    if not isinstance(loaded, dict):
+        print(f"  ! {path.name}: not a mapping, refusing to overwrite it")
+        return None
+    return loaded
+
+
+def write_catalog(path, catalog):
+    """Replace ``path`` with ``catalog`` in one step.
+
+    Writing in place would leave a half-written catalogue behind if the process
+    is interrupted or the disk fills, and the truncated remainder can still be
+    valid YAML — the next run would then machine-fill the destroyed entries.
+    """
+    temporary = path.with_name(f"{path.name}.tmp")
+    temporary.write_text(
+        yaml.safe_dump(catalog, allow_unicode=True, sort_keys=True, width=1000),
+        encoding="utf-8",
+    )
+    os.replace(temporary, path)
+
+
+def translate(session, url, api_key, text, target):
+    """Translate one string into ``target``, or return None on failure."""
+    payload = {
+        "q": text,
+        "source": i18n.SOURCE_LANGUAGE,
+        "target": target,
+        "format": "text",
+    }
+    if api_key:
+        payload["api_key"] = api_key
+
+    try:
+        response = session.post(
+            f"{url}/translate", data=payload, timeout=REQUEST_TIMEOUT
+        )
+    except requests.RequestException as error:
+        print(f"  ! {target}: {type(error).__name__}, {error}")
+        return None
+
+    if not response.ok:
+        print(f"  ! {target}: {response.status_code} {response.text[:120]}")
+        return None
+
+    try:
+        translated = response.json().get("translatedText")
+    except ValueError:
+        print(f"  ! {target}: answered 200 but not JSON")
+        return None
+
+    if not isinstance(translated, str) or not translated.strip():
+        print(f"  ! {target}: unusable translatedText ({translated!r})")
+        return None
+    return translated
+
+
+def sync(url, api_key, sources, targets, directory, keep=()):
+    """Fill and write the catalogue of every language in ``targets``.
+
+    Args:
+        url: base URL of the LibreTranslate instance.
+        api_key: API key, or an empty string.
+        sources: English strings to translate.
+        targets: language codes to fill.
+        directory: catalogue directory to write into.
+        keep: strings to store as themselves instead of translating.
+    """
+    directory.mkdir(parents=True, exist_ok=True)
+    session = requests.Session()
+
+    available = supported_languages(url, session)
+    for target in targets:
+        if target not in available:
+            print(f"- {target}: not offered by {url}, skipped")
+            continue
+
+        path = directory / f"{target}.yaml"
+        catalog = load_existing(path)
+        if catalog is None:
+            continue
+
+        protected = {word: word for word in keep if word not in catalog}
+        catalog.update(protected)
+
+        shipped = (
+            {}
+            if directory == i18n.UI_DIR
+            else i18n.read_catalog(i18n.UI_DIR / f"{target}.yaml")
+        )
+        missing = sorted(
+            source
+            for source in sources
+            if source not in catalog and source not in shipped
+        )
+        if not missing:
+            print(f"- {target}: complete")
+            continue
+
+        print(f"- {target}: translating {len(missing)} string(s)")
+        added = 0
+        for source in missing:
+            translated = translate(session, url, api_key, source, target)
+            if translated:
+                catalog[source] = translated
+                added += 1
+
+        if not added and not protected:
+            print(f"  ! {target}: nothing translated, leaving the file untouched")
+            continue
+
+        try:
+            write_catalog(path, catalog)
+        except OSError as error:
+            print(f"  ! {target}: could not write {path.name}: {error}")
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "--url",
+        required=True,
+        help="Base URL of a LibreTranslate instance, e.g. http://localhost:5002",
+    )
+    parser.add_argument(
+        "--api-key",
+        default="",
+        help="API key, if the instance requires one.",
+    )
+    parser.add_argument(
+        "--config",
+        type=Path,
+        default=DEFAULT_CONFIG_PATH,
+        help=f"Configuration to read the prose from (default: {DEFAULT_CONFIG_PATH}).",
+    )
+    parser.add_argument(
+        "--catalog",
+        choices=("content", "ui"),
+        default="content",
+        help=(
+            "content: your configuration's prose, generated per deployment. "
+            "ui: the interface strings that ship with the project."
+        ),
+    )
+    parser.add_argument(
+        "--keep",
+        nargs="+",
+        default=[],
+        metavar="STRING",
+        help=(
+            "Store these as themselves instead of translating, "
+            "in addition to --keep-file."
+        ),
+    )
+    parser.add_argument(
+        "--keep-file",
+        type=Path,
+        default=DEFAULT_KEEP_PATH,
+        help=(
+            f"One string per line, stored untranslated (default: {DEFAULT_KEEP_PATH})."
+        ),
+    )
+    parser.add_argument(
+        "--languages",
+        nargs="+",
+        default=[code for code in i18n.LANGUAGES if code != i18n.SOURCE_LANGUAGE],
+        help="Language codes to fill (default: every shipped language).",
+    )
+    args = parser.parse_args(argv)
+
+    if args.catalog == "ui":
+        directory = i18n.UI_DIR
+        sources = set(i18n.UI_STRINGS)
+        print(f"{len(sources)} interface string(s)")
+    else:
+        directory = i18n.CONTENT_DIR
+        config = yaml.safe_load(args.config.read_text(encoding="utf-8"))
+        sources = collect_sources(config) | set(i18n.UI_STRINGS)
+        print(f"{len(sources)} string(s) in {args.config} and the interface")
+
+    keep = read_keep(args.keep_file) + args.keep
+    print(f"{len(keep)} string(s) kept untranslated")
+
+    try:
+        sync(
+            args.url.rstrip("/"),
+            args.api_key,
+            sources,
+            args.languages,
+            directory,
+            keep,
+        )
+    except BackendError as error:
+        print(f"ERROR: {error}", file=sys.stderr)
+        return 1
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())