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/i18n/supportedLocales.js

@@ -0,0 +1,23 @@
+/**
+ * Zentrale UI-Locales (BCP 47). Reihenfolge für Dropdowns (Deutsch zuerst, dann üblich).
+ */
+export const SUPPORTED_UI_LOCALES = ['de', 'en', 'ceb', 'es', 'fr'];
+
+/** hreflang / SEO: ohne funktionale Abweichung zu {@link SUPPORTED_UI_LOCALES}, nur Reihenfolge wie bisher in seo.js. */
+export const SEO_UI_LOCALES = ['de', 'en', 'es', 'ceb', 'fr'];
+
+export const HREFLANG_FOR_UI = {
+  de: 'de',
+  en: 'en',
+  es: 'es',
+  ceb: 'ceb',
+  fr: 'fr',
+};
+
+export const OG_LOCALE_FOR_UI = {
+  de: 'de_DE',
+  en: 'en_GB',
+  es: 'es_ES',
+  ceb: 'ceb_PH',
+  fr: 'fr_FR',
+};