Nope, it is not possible. People have often asked for this but realizing it would apparently mean a lot of work and eat up a lot of resources as well. The developers decided to focus on a second full English release instead. Seems unlikely that a German subtitle version will be made in the future.
Also, I remember another user saying that he was very confident in his conversational German. But then he was frequently stumped by the language used in the game. So I guess it's not the best way to learn or to test your ability anyway.