feat(i18n): serve every page in 30 languages

The interface ships translated; page content stays English until a
LibreTranslate instance fills app/i18n/content/ through make i18n. A string
without a catalogue entry falls back to its English source, so a half-filled
catalogue degrades instead of breaking.

Translation runs after ConfigurationResolver.resolve_links(), on a copy.
resolve_links matches by the `name` field, so translating it beforehand
would break every `link:` reference in the configuration.

negotiate() normalises to the primary subtag itself. Werkzeug's best_match
returns an exact match before it considers a primary-tag fallback, so the
Chrome default `de-DE,en;q=0.8` resolves to English there. "/" carries
Vary: Accept-Language, without which a shared cache pins the first
visitor's language for everyone.

The route rule lists the known codes as a converter argument. A bare
"/<lang>/" answers /robots.txt and /favicon.ico with a permanently
cacheable 308 to their trailing-slash form.

Templates gain lang, dir, the RTL stylesheet, a canonical URL and 30
hreflang alternates. Those are the first external URLs in this app:
ProxyFix takes the scheme from X-Forwarded-Proto so they do not claim
http:// behind a TLS-terminating proxy, X-Forwarded-Host stays untrusted
because nginx passes a client-supplied one through, and TRUSTED_HOSTS lets
Flask reject a forged Host outright.

Flask only autoescapes .html/.htm/.xml/.xhtml/.svg, so every *.html.j2
template interpolated configuration raw. Enabling it changes two lines of
the shipped page, both an apostrophe.

read_catalog degrades an unreadable catalogue to English rather than
serving a 500, and drops non-string entries that would otherwise render as
"42". i18n_sync writes atomically, never overwrites an existing entry,
refuses to touch a catalogue it could not parse, and leaves the file alone
when a run translated nothing.

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

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_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:
-            data = resp.json()
-            if data.get("media_type") == "image":
-                apod_bg = data.get("url")
+    except requests.RequestException:
+        logging.warning("APOD lookup failed", exc_info=True)
+        return None
+
+    if not resp.ok:
+        return None
+
+    data = resp.json()
+    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 +1,228 @@
+// 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 all thirty', () => {
+    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', 30);
+  });
+
+  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('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', 31);
+    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/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/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/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">
+                    {% 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 +1,163 @@
+"""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
+
+I18N_DIR = Path(__file__).resolve().parent.parent / "i18n"
+UI_DIR = I18N_DIR / "ui"
+CONTENT_DIR = I18N_DIR / "content"
+
+SOURCE_LANGUAGE = "en"
+
+LANGUAGES = {
+    "en": "English",
+    "zh": "中文",
+    "hi": "हिन्दी",
+    "es": "Español",
+    "fr": "Français",
+    "ar": "العربية",
+    "bn": "বাংলা",
+    "pt": "Português",
+    "ru": "Русский",
+    "ur": "اردو",
+    "id": "Bahasa Indonesia",
+    "de": "Deutsch",
+    "ja": "日本語",
+    "tr": "Türkçe",
+    "ko": "한국어",
+    "vi": "Tiếng Việt",
+    "it": "Italiano",
+    "th": "ไทย",
+    "pl": "Polski",
+    "nl": "Nederlands",
+    "uk": "Українська",
+    "fa": "فارسی",
+    "ro": "Română",
+    "el": "Ελληνικά",
+    "cs": "Čeština",
+    "sv": "Svenska",
+    "hu": "Magyar",
+    "he": "עברית",
+    "da": "Dansk",
+    "fi": "Suomi",
+}
+
+RTL_LANGUAGES = frozenset({"ar", "fa", "he", "ur"})
+
+AUTOFILL_KEYS = frozenset({"description", "text", "warning", "info", "subtitel"})
+
+TRANSLATABLE_KEYS = AUTOFILL_KEYS | frozenset({"name", "title"})
+
+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}