Well from my knowledge just like Saigor said german is a requirement fro all ACC type facilities in Germany. The only exception i could see would be the major airports but well then again i'm not quite aware of the operations but everytime i listened to Fra app it was most of the time in English but that is just the times i was able to listen.
However when you think of rhine radar well overall it is just like eurocontrol. All Eu members can apply since all traffic is conducted in the english language which makes things easier to find candidates.
So therefore of course when DFS opened slots for foreign controllers well everybody assumes that the placement will be in Karlsruhe or maybe Bremen (i think that is were the north UAC is right???).
But overall just by respect i believe that who ever applied for this oppurtunity should take in consideration that down the road an intensive class in German might be a good idea. It would be in my case....plus it would make the lady happy hehe