✨(app) get language from backend; set browser-detected language if null

- adds useLanguageSynchronizer hook to update the:
  1. frontend-language to the user-preference - if there is one.
  2. user-preference to the (browser-detected) frontend-language - otherwise.

CHANGELOG.md

@@ -20,6 +20,7 @@ and this project adheres to
 - 🔒️ Manage unsafe attachments #663
 - ✨(frontend) Custom block quote with export #646
 - ✨(frontend) add open source section homepage #666
+- ✨(frontend) synchronize language-choice #401
 
 ## Changed