Not much help with sewing classes offered in English either, but can recommend two places in the area. You'd definitely need some German to get by though.
I did a dress-design course several years ago at a fabulous place in Offenbach, called
Schule für Mode, Grafik und Design. It was great fun, I learnt a lot and gained so much confidence that I hardly ever buy patterns anymore and can now turn out funky and unique kids' clothing at alarming speed. They also offer beginner and advanced sewing classes as well as a whole bunch of drawing, painting and design courses.
Another great place is
Martino-Stoffe in
Sachsenhausen. They offer sewing classes for all levels, weekend and kids' courses as well as little refresher workshops. Groups are small, six people maximum. The ladies are lovely and extremely helpful, I go there whenever I'm stuck and in need of advice and inspiration.