No UI / way to set a language?

I just installed the APK, and when opening it, there is no UI and there is generally no way to set a language or anything.

I even downloaded the German language model manually. But where do I put it?

(My ultimate goal is to create my own voice personalized assistant, specifically just by using a *offline* TTS engine, a shell script (in Termux) and an adapter to search engines (which adds a bit of semantic information). But having offline voice recognition on Android in general is of course the best way to start.)