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/App.vue

@@ -65,7 +65,13 @@ export default {
     MultiChatDialog,
   },
   created() {
-    this.$i18n.locale = this.$store.getters.language;
+    const code = this.$store.getters.language;
+    const loc = this.$i18n.locale;
+    if (loc && typeof loc === 'object' && 'value' in loc) {
+      loc.value = code;
+    } else {
+      this.$i18n.locale = code;
+    }
   },
 };
 </script>