I think this highly depends on your intentions with communicating with the foreigner, the foreigner's native language, and how comfortable you are with speaking the foreigner's native language. If you're in a hurry and you need to communicate something to a foreigner, English would be the best bet, since English is considered to be an international language. Because if you speak in Chinese and he doesn't understand you (which is highly likely), you'll have to repeat what you said in English. Also, if you know that the foreigner doesn't speak Chinese, speaking to him in Chinese wouldn't make much sense.

The chance of a foreigner speaking Chinese is pretty much nil, so communicate in English if you want to be understood.

The one thing I will say is absolutely easy to decide is this: if a foreigner comes up to you and starts speaking Chinese, you should respond in Chinese. If a foreigner comes up to you and starts speaking English, you should respond in English. As for other cases, I disagree with the other answers here, but of course you are welcome to make up your own mind. I think you should speak in Chinese first, and try English if they don't understand you. The reason I say this is because it can be a little bit racist if you assume that no foreigner speaks Chinese, and every foreigner speaks English. I think it's more polite to assume that any person, no matter what they look like, has put in the work to try and learn the language of the country where they're living. I personally am studying Korean, because I want to live in Korea, and I get really nervous about this. I worry that every Korean I meet will speak to me only in English, and that would make me feel sad. It would show me that they think I don't belong, or they think I'm not smart enough to speak Korean. I think that foreigners in China will feel the same way.

Well you should, certainly, because you may find that it is much easier. If you are very careful about picking a language partner, you will find someone that is very easy to talk to and you may find that they can help you to feel relaxed while you practice.