I'd guess Dutch, though there seem to be quite a few people who speak German here as well. I believe it's a second language for most though.

Correct me if I'm wrong, but I believe the active members who speak it as a first language are Husar, Ser, and myself, plus a few less active members or members who don't advertise their location (or ones I forgot...sorry ).

Add that to the fair amount who seems to have a knowledge of German from anywhere from basic to fluent, and we have quite a few members.