if the two speaks different language, the most common language to use is of course English. If you're not that fluent you can just say little words and mix it with sign languages or actions. For example, if you are asking for the time, you can just say the word "time" and then point at your wrist. It's understandable.

I mean that when the two speak different language ，how to chat？

What do you mean by "express the language"?