When chatting to a person who doesn't have the same native language, should you write in clear, formal language or is it useful to use colloquialisms?

The person I’m chatting with is about 18, so his grasp of the English language is good, but it’s not his native language. I don’t have any grasp of his language, so I feel that translating a colloquialism may be beneficial to him sometimes and at other times, not so beneficial. However, I have no way to judge that, and I don’t want to insult him. If you’ve experienced this, do you have any suggestions?