Hello there. If you go to about:preferences#content in your browser. And scroll down to languages, are you allowed to add German to your list of accepted languages?

i can add german to the languages list, i can put it in 1.st position but this does noct change the interface. Klaus

HI   !

You might be better off downloading Firefox in your own language from here :

https://www.mozilla.org/en-US/firefox/all/

You could also take a look at this article :

Use the Firefox interface in other languages with language packs