Sorry I cannot help but I do know of plenty in Frankfurt.
You could try and find a local Sportverein yes the classes will be in German but I know quite a few people here who do not speak much German but still manage to get through classes.
I also found this place
Sarastudio which is a pilates and yoga studio. It looks pretty nice but again in German. But like I say you might find it not hard to follow and it'll help you improve your German. I go to step and find all the steps have the same names in German (mambo, grapevine, basic etc) so you wouldn't need much German to follow it just the numbers to keep up.
Hope this helps