At first you'd think that local People don't go to plays in English, which is not true for such an international city like Frankfurt. But of course, you will also find a lot of English-speaking expats here. The stagings are so good, that some of them even get played in London Theatres as well.
It's the biggest English Theatre in Europe (apart from the ones in UK/Ireland of course). They have about 300 seats and have a mixed repertoire of classics and modern plays. The actors usually come directly from England or other English-speaking countries.
It's better to get the tickets well in advance, because it's often fully booked!