Multilingual recognition and responses

[JosXa]

If I tell Alexa to "reply to me in English and German" or set multiple languages up in the Alexa App, it can automatically adjust its recognized languages and also answer me accordingly. However, this appears to be a client-side setting, as no matter which LANGUAGE or TTS_LOCALE environment variable I set for the CLI, I just cannot get it to answer me in English. So far, I've only tried the alexa.amazon.de domain though to avoid having to log out and in again constantly, which wouldn't really be an option.

Has anyone gotten text recognition and responses to work in multiple languages?
Or, to lessen my requirements, has anyone managed to get their Alexa to understand English in their non-English speaking alexa.amazon.* region at all (, sacrificing your own language)?

If so, is that a matter of environment variable settings or something completely different?