It occurs to Brainy that one, the creature clearly can't see him, and two, it actually is trying to speak. Unfortunately he doesn't recognise it or anything it might be related to at all, so the chances of him knowing a language that might be more familiar are extremely low.

He takes a risk in gently patting its head, hoping that isn't going to offend it.

"Can you understand me even if you can't speak very well?"