feat(i18n): add French language support and enhance localization

- Introduced French as a supported language across the application, updating locale files and adding translations for various components.
- Enhanced language handling logic to accommodate French, ensuring proper detection and fallback mechanisms.
- Updated UI elements to include French language options, improving accessibility for French-speaking users.
- Refactored SEO handling to include French in hreflang links, enhancing search engine indexing for multilingual content.
- Added new scripts for managing French translations and ensuring consistency across language files.

frontend/src/utils/seo.js

@@ -1,25 +1,17 @@
 import { getPublicBaseUrl } from './appConfig.js';
+import {
+  SEO_UI_LOCALES,
+  HREFLANG_FOR_UI,
+  OG_LOCALE_FOR_UI,
+} from '../i18n/supportedLocales.js';
 
 const DEFAULT_BASE_URL = 'https://www.your-part.de';
 const DEFAULT_SITE_NAME = 'YourPart';
 const DEFAULT_TITLE = 'YourPart - Community, Chat, Forum, Vokabeltrainer, Falukant und Minispiele';
 const DEFAULT_DESCRIPTION = 'YourPart verbindet Community, Chat, Forum, Blogs, Vokabeltrainer, das Aufbauspiel Falukant und Browser-Minispiele auf einer Plattform.';
 const DEFAULT_IMAGE = `${DEFAULT_BASE_URL}/images/logos/logo.png`;
-const SEO_UI_LOCALES = ['de', 'en', 'es', 'ceb'];
 
-const HREFLANG_FOR_UI = {
-  de: 'de',
-  en: 'en',
-  es: 'es',
-  ceb: 'ceb',
-};
-
-const OG_LOCALE_FOR_UI = {
-  de: 'de_DE',
-  en: 'en_GB',
-  es: 'es_ES',
-  ceb: 'ceb_PH',
-};
+export { SEO_UI_LOCALES, HREFLANG_FOR_UI, OG_LOCALE_FOR_UI };
 
 const MANAGED_META_KEYS = [
   ['name', 'description'],
@@ -138,7 +130,7 @@ function uiLocaleToOgLocale(ui) {
   return OG_LOCALE_FOR_UI[ui] || OG_LOCALE_FOR_UI.de;
 }
 
-/** OpenGraph locale (z. B. de_DE) aus UI-Sprache (de|en|es|ceb). */
+/** OpenGraph locale (z. B. de_DE) aus UI-Sprache (de|en|es|ceb|fr). */
 export function seoOgLocale(ui) {
   return uiLocaleToOgLocale(ui);
 }