"You don't have to write your words, I can understand sigh language." Bruce stops you before you could write the next word. "Or perhaps you don't know?" He asks as he raises his brow.

It's a possibility that you might not know sign language, since Gotham orphanages don't really care about mute kids. Or you might think nobody cares enough to learn something you communicate with. Bruce does care, he wouldn't adopted you if he didn't.