Describing a person that can speak Mandarin
I just want to expand on what is being said in the comments about context. One of the problems of speaking a language other than your first language is getting across the point of what you are trying to say, not just saying your main point but making yourself fully understood.

Think about this in the context of English. If you were going to say "Yesterday I met someone who could speak English". If you were saying the sentence in English to someone else who understood English, in most cases it would have the listener wondering what your point was. Especially in this case as you are the foreigner, in terms of speaking Chinese, you are the special case.

So maybe you want to say "I met another foreigner who can speak Chinese" or "I met a Chinese person and I was able to practice my Chinese speaking with them".

I understand this doesn't answer directly what you are asking, but getting your real point across is an important step in going from learner to expert. 

When I just started out learning my wife used to say to me, stop for a second, tell me in English what you really want to say and then start your sentence again.