They know a native tongue amongst each other and a 'common' tongue i.e. what everyone in Amani is perceived to be speaking. They are also quick to learn enough of other languages to converse basic ideas when necessary.

They'd likely find sign language to be much more convenient than anything though. It would give them the ability to speak with land-dwellers when they can't speak beneath the water. So, yes, he can teach them and they would enjoy it!