Sure.

Case: Imagine you are a native Spanish speaker and so is your customer and let's say you are an expert in finance. You can choose to teach in Spanish, of course. Why not?

Case: Imagine that you wanted to teach French and your Student's mother tongue is Japanese. That should be possible.
But watch out!

Make sure to set a required minimum English level on your Course so that your Japanese customer can communicate with you in case they can't express themselves in French. There must always be a common language for you to communicate with your customers. And as a Teacher, that is your responsibility.
Note: you can also set up to 4 different languages on your Course to attract customers who speak those languages natively.