Compliment them in Chinese, if they don't understand then don't compliment them yet. I think most foreigners feel patronized especially if they are aware that the fawning is disingenuous.

It depends who your talking too, I suppose. "Wow..." is more likely to be used in the USA, than UK "You speak Mandarin, very well." or "You understand Mandarin well" or "Your Mandarin, is great"

answer:　Your Mandarin speaking is quite good *that's the only one what I know:)