As German is a language not too many staff members might speak, the forum staff may find it difficult to judge if what is going on in your thread is against any of the forum rules or not.
But ... if that is the only hindrance I would volunteer to translate where necessary and keep an eye on things, should that be required to swing staff opinion in favour of allowing it.
That aside I don't think there are any rules here that would prevent you from playing a game by private message in whatever language you choose, but writing in English might be the better choice for a forum game, simply because it allows you to create "writing samples" that may help you to attract other roleplaying partners in the future. The moment you start writing in a language that is alien to most people on the forum you create your own little niche and it may be difficult to break out of that, once people see you as someone who would rather write in your own language than something they can understand too.
