I wrote "The woman has a suit on" and it was marked incorrect. Reported 22-Nov-2014
That is really very strange. The machine tells me that is an accepted answer. You don't happen to have a screenshot? I see that The woman has on a suit is missing, and that one has been reported, so I'll add that. But I can't add what is already accepted. If you or anyone else who stops by has this problem again, please try to upload a screenshot so we can report it to staff.
I think suit is men's cloth, and the 'kostym' is dress in english for women.
kostym always means "suit", regardless of the gender of its wearer. A dress is a klänning.
Does this mean that it can also be a skirt with matching jacket for a woman? A "mantelpakje" in Dutch.
Yes, it can have that meaning, at least as far as dress codes go.
That isn't really an idiomatic English construction, nor should it be "costume" but "suit".
Often also kostym - it's a little problematic, but we don't want to accept it since we don't want people to learn the wrong thing.