Hi Bussibär,
The information page stated that it was English only, and that is what you received. It is true that the German option does not work correctly, but that is because the game is - as the information page says - English only. The German option wouldn't have been visible to you until you purchased the game and played around with the settings, so I am curious how you thought there was a German translation available before you purchased it. Obviously there was confusion somewhere. If you could tell me how you came to that conclusion, that would help me a lot.
Unfortunately, I am not in a position to give you your money back since you bought it from Steam. If you would like a refund, you will need to contact them.
-Dave