Métis people have their own languages and dialects, but there is no single "Metis" language. They often speak French, English, or other languages of their surrounding communities.
If you are trying to greet someone from the Métis community, it's best to use the language they speak. However, if you're unsure, a simple "Hello" or "Good morning/afternoon" would be appropriate.
You can also research the specific Métis community you're interested in to learn more about their language and culture.