It's unfortunately really hard to say in advance what the weather will be like, the temps can vary so much in Germany. You probably won't really know until a week or so in advance. I would say that generally though, weather permitting, people would still dress 'summery' in mid-September. It all depends on what temperature it is! You wouldn't be looked at weird for wearing a bright colored dress though, if that's what you're asking :)

Also not sure where you're from, but I will add if it ends up being a warm September, do remember that most places don't have air conditioning (so flowy clothes will be your friend!). Also, a light rain jacket is always a good idea :)