In the WoW - Community there are so many words that are adapted from both, english - german and vice versa.
I don't think that you will have to much of a stuggle to communicate with a decent amount of players on this server at all.
Just consider that regardless of the english skills that most germans do have, it'll be more difficult to communicate certain requirements for raids and groups. It will be inevitable for you to clearly communicate with raid leaders that you need more specific or detailed explanations in their raids in order to fufill the job you may be appointed to.
Other than that, I think it will be a great opportunity to learn some German, although it will be a "gamer-German", you can always learn something new.
If you got any questions of any kind, please don't hesitate contacting me ingame.
Although I am a native german speaker I live in the United States and might have some tips and answers to some of your questions that might rise up at some point.
Welcome to the Server and the Community !!
Best,