Halp!

So I'm about to make a game, and the guy I'm working with wants to write it in Sepples! What do I do?

You say that you're working with this guy, so I don't see how "wanting" has anything to do with it. If you can't agree amongst yourselves, your boss should pick a language. This language should be whatever will get a saleable product quickest--even if neither of you know it yet.