feat(localization): expand language support and enhance UI for user settings

- Added support for additional UI locales including Cebuano and Spanish, improving accessibility for a broader user base.
- Updated language selection components in the AppHeader and SettingsWidget to reflect new language options, enhancing user experience.
- Enhanced localization of various UI elements across components, ensuring consistent language representation and improved user engagement.
- Implemented logic to synchronize user language preferences with backend settings, providing a seamless experience when changing languages.

frontend/src/main.js

@@ -51,12 +51,26 @@ function getBrowserLanguage() {
     return 'en';
 }
 
+const SUPPORTED_UI_LOCALES = ['de', 'en', 'ceb', 'es'];
+
+function getInitialAppLanguage() {
+    try {
+        const saved = localStorage.getItem('uiLanguage');
+        if (saved && SUPPORTED_UI_LOCALES.includes(saved)) {
+            return saved;
+        }
+    } catch (_) {
+        /* ignore */
+    }
+    return getBrowserLanguage();
+}
+
 const vuetify = createVuetify({
     components,
     directives,
 });
 
-store.dispatch('setLanguage', getBrowserLanguage());
+store.dispatch('setLanguage', getInitialAppLanguage());
 
 const app = createApp(App);