feat(i18n): add internationalization support with English and German translations

- Integrated i18next for language detection and translation management.
- Added translation files for English and German languages.
- Implemented translation keys for notifications, reminders, and common UI elements.
- Updated main application entry point to include i18n initialization.
- Styled language selection dropdown in settings.
- Enhanced package dependencies to include i18next and react-i18next.

frontend/src/i18n/index.ts

@@ -0,0 +1,30 @@
+import i18n from 'i18next';
+import { initReactI18next } from 'react-i18next';
+import LanguageDetector from 'i18next-browser-languagedetector';
+
+import en from './en.json';
+import de from './de.json';
+
+const resources = {
+  en: { translation: en },
+  de: { translation: de },
+};
+
+i18n
+  .use(LanguageDetector)
+  .use(initReactI18next)
+  .init({
+    resources,
+    fallbackLng: 'en',
+    supportedLngs: ['en', 'de'],
+    interpolation: {
+      escapeValue: false, // React already escapes
+    },
+    detection: {
+      order: ['localStorage', 'navigator'],
+      caches: ['localStorage'],
+      lookupLocalStorage: 'medassist-language',
+    },
+  });
+
+export default i18n;