Irina, welcome at the Mudcat! I'm afraid, though, I'm not much help with questions about dance music. We have a lot of folk dancing in Northern Germany, but as the local pieces are often very staid bands prefer international dances. As to instruments, accordion or button accordion and hurdy-gurdy seem to be very popular, but this needn't mean they were in former times.
If you're interested, join the 'folkmail' mailing list mailto:folkmail@folkclubfrankfurt.de web interface for joining etc.: http://www.folkclubfrankfurt.de/majordomo/.
There are quite a few dance enthusiasts among the members.