frisk initially spoke english and very basic asl they picked up from their tutor! they’re not hard of hearing and their vocal apparatus is pretty much fine, they’re just really slurry/muttery unless they put a ton of effort into it and dont usually have the spoons to spare. when they landed in the underground they chomped the shit out of their tongue, so they stuck to just signing until they felt better and by the time that happened the word was out they were mute and everyone was so nice and accommodating and they’d gotten so fluent they felt too awkward to correct anyone about it lmao. it took years for them to slip up and talk to themselves in public
they become multilingual thru their work as an ambassador later on but they still prefer to stick to writing and having an interpreter follow them around