i'm getting diffrent language for each nation atla au brain worms, here is my HC
Sokka and Katara both only speak their native lanuage, but neither of them can write or read very well, that village looks poor poor i'd doubt they had time to make the kids had a good education, also war
*South and Northen tribes have slightly diffrences, think Englisj and Australia type of stuff(South is the Australia) swap people talk like red neck Americans
Zuko as a price had tutors so he speaks Fire nation and Earth kingdowm fluently same with reading them, he knows some water nation but he was bannished before getting any more lessons he also cannot read it
*Fire nation language is the german of the world, they sound angry and agressive
Toph she is fluent in Earth kingdom, she knows some fire nation and water tibe, but she wasn't really paying attention when taught by teachers her parents hired(she can't read also)
*Earth kingdom has a bunch of dylects due to being a large country, for example desert people have their own dylect
Suki here where my head canon come in, Kyoshi is an insolated island, and i think their language would be a mix between Earth, Fire and Water languages
*why? They got Earth from being taken away Earth kingdom Fire nation from Rangi(passed it down) and Water tribe from the fact they wear blue
Now the best for last
Aang, due to being a nomaid and the avatar he has been taught all of the languages, but by the time he's out, it's outdated as hell, he can bearly communicate with anyone else
*Since Aang is the last native speaker, his children don't learn true Air nation language, instead any quirks of Aang get officialized into the language,(example me and my siblings have a habit of saying a certain French store as it's written, we've been doing to for years)









