Yeah, Amazon reminds me of Google a few years ago: Language settings are a clusterfuck. My Amazon.de is in English, Alexa and generally digital accounts are connected to Amazon.de (otherwise I'd lose prime video and prime music without paying twice).

English books work fine, but for some reason, I can't add skills while Alexa is in English. I need to switch it to German, add skills, switch it back to English for it to work.