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/package.json

@@ -10,8 +10,11 @@
     "lint": "echo 'add lint config'"
   },
   "dependencies": {
+    "i18next": "^24.2.2",
+    "i18next-browser-languagedetector": "^8.0.4",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
+    "react-i18next": "^15.4.1",
     "react-router-dom": "^7.11.0",
     "zod": "^3.23.8"
   },