!! honestly i imagine them to speak a language similar to korean but very different still. i think i mentioned it in an authors note at an end of chap maybe in 8 (? idk lol) but they both speak the same language, but their ways of writing it is different. think of goryeo and the earlier joseon period where they wrote in hanja but had different tongue to chinese, it is basically that but flipped; they speak the same language with different writing scripts developed away from each other