feat: add German and English UI language support

A single-language interface makes the app less usable for part of the audience
and encourages more hardcoded copy as new screens and states are added.
Introducing a shared i18n path with a persisted language preference gives the
UI one consistent way to present text and lowers the risk of partial or
inconsistent localization over time.

The surrounding copy cleanup belongs in the same change because it aligns
existing labels, messages, and settings text with that translation structure
instead of leaving mixed localization approaches in place.

.env.example

@@ -1,10 +1,15 @@
 # Eigene Twitch Application registrieren auf https://dev.twitch.tv/console/apps
+# Register your own Twitch application at https://dev.twitch.tv/console/apps
 # - Name: beliebig (z. B. "Twitch4SteamDeck dev")
+# - Name: any value (for example "Twitch4SteamDeck dev")
 # - OAuth Redirect URLs: http://localhost  (wird beim Device Code Flow nicht benutzt,
 #   das Feld muss aber gesetzt sein)
+# - OAuth Redirect URLs: http://localhost (not used by the Device Code Flow,
+#   but the field must still be set)
 # - Category: Application Integration
 # - Client Type: Public
 #
 # Danach die Client-ID hier eintragen und die Datei in `.env` umbenennen.
+# Then add the client ID here and rename the file to `.env`.
 
 MAIN_VITE_TWITCH_CLIENT_ID=