I can't seem to change the Apple TV language on my 4K flat screen (using Apple TV streaming) from German to English (US).

The Apple TV device has an interface language, independent of the content languages. Two different settings.

Content has languages (audio, subtitles) as provided for that content, which may differ per title. English may not always be available in Germany. The Apple TV device allows to set a preferred audio language and/or subtitle, if available for that content.

Change the language on your Apple TV - Apple Support

Change the subtitles or audio language for the Apple TV app and iTunes Store movies and TV shows - Apple Support

Adjust video and audio settings (defaults, if available) on Apple TV - Apple Support

Apple TV+ content, including Greyhound, is available in many audio languages and subtitles in all countries and regions. (I wish more studios did that.) I can watch e.g. with Japanese audio and Chinese subtitles, even if I don’t understand any of that, and those languages originate 5000 miles from where I am.

1. While watching a video, swipe down on your remote's Touch surface.
2. Swipe right to see the Subtitles or Audio menus.
3. Swipe down and choose the language or subtitles that you want to use.