I think one should be able to get away with 'is your girl...'. Dziewczyna does not necessarily mean a girl friend anyway . But then my way sounds like ownership which is a definite no no . Well, it's just an observation.
I can't get the difference between your girl and your girlfriend, I believe boyfriend is kawaler?
"dziewczyna" simply means "a girl" (a teenager/a young woman), unless it's "someone's dziewczyna", then she turns into a girlfriend.
"kawaler" means a bachelor, an unmarried man. As something similar to "a boyfriend", it seems rather dated to me. But it is something that my grandma could say.