I agree with Ryoko-san.
If you look like a person who are living outside Japan, many people here would say 日本語、話せますか? because they think it's natural that you don't speak Japanese.
If you look like a person who it's unnatural to speak Japanese in some reasons, they would say 日本語、話しますか?, but it sounds unnatural a bit. I would say 話していますか? as Ryoko-san said.
Or some people say 日本語、話しますか? to anybody because they know some foreigners who know Japanese a little would feel bad when they are asked 話せますか (Can you ~?). Those foreigners insist that "Can you" is rude because they feel the speaker would think they have no ability. (I think it's ridiculous, though. When people here say 話せますか, it doesn't have such a meaning)