upgrade project to be multilanguage

[dmm1]

Titel:
Upgrade project to support multilingual frontend with automatic language detection

Beschreibung:
Enhance the frontend to support multiple languages (i18n) with the following goals:

🎯 Objectives:
Add multilingual support (initially: English + German; extensible).

Automatically detect the user's browser language on first visit.

Store the preferred language in local storage or cookies for future visits.

Allow manual language switch via a UI dropdown or setting.

Ensure all UI strings are managed via translation files (e.g. JSON or i18next).

Fallback to English if a translation is missing or detection fails.

✅ Acceptance Criteria:
Language is auto-detected from the browser's navigator.language.

User can manually override language preference.

Translations are loaded from modular translation files.

UI elements and error messages reflect selected language.

Fallback mechanism to English is in place.

Works in both web and native (React Native) environments.

📦 Suggested Stack:
i18next or react-i18next for internationalization