refactor(i18n): improve language handling and UI updates

- Enhanced language setting logic in App.vue and AppHeader.vue to support reactive updates without page reloads.
- Updated language options in AppHeader.vue to use native labels for improved clarity.
- Introduced a utility function in i18n/index.js to streamline locale updates, ensuring consistent language handling across the application.

frontend/src/i18n/index.js

@@ -195,11 +195,20 @@ const i18n = createI18n({
   messages
 });
 
-// Überwache Änderungen der Sprache im Store und aktualisiere i18n entsprechend
+function setGlobalLocale(next) {
+  const loc = i18n.global.locale;
+  if (loc && typeof loc === 'object' && 'value' in loc) {
+    loc.value = next;
+  } else {
+    i18n.global.locale = next;
+  }
+}
+
+// Überwache Änderungen der Sprache im Store und aktualisiere i18n (ohne Seiten-Reload)
 store.watch(
   (state) => state.language,
   (newLanguage) => {
-    i18n.global.locale.value = newLanguage;
+    setGlobalLocale(newLanguage);
   }
 );