Hello S-C-L.
Thank you for using Apple Support Communities. It sounds like you're experiencing issues with one specific app when using your Apple TV. Specifically, all language coming from the app is in German. If so, we'd like to look into this issue with you.
You've already done a great job in troubleshooting this specific issue. Reinstalling the app is a great step to attempt. It's highly possible that is is occurring due to being located in Germany. However, we want to make sure we point you in the right direction for help. If your Apple TV is fully updated and you've changed all settings for English within this specific app, we recommend that you reach out to the app developers for assistance. The following resource can show you how to reach them:
How to contact an app developer
We hope this information is useful. Best of luck.