Me holding a dvd player; "This is Avatar; The Last Airbender! It's one of the most beloved shows of the future for its storytelling and thorough worldbuilding!"
J. R. R. Tolkien whom I time traveled to meet; "Why does everyone speak the same language?"
Me; "Well... maybe they are speaking different languages, but it was all translated to english?"
Tolkien; "Then how do they all understand each other if they are speaking different languages?"
Me a bit defensive and nervous; "Uh... the Fire Nation is impirial and nationalistic. Maybe they forced their language on the world, and that's what everyone's speaking."
Tolkien; "Impossible. Ba Sing Se would never speak the language of a country they don't know is attacking. And the Northern Water Tribe would not speak the language of conquerors who are at their doorstep."
I feel like the answer to this is the Avatar system. Though the world of Avatar contains many sovereign nations they are all connected by the institution that is the Avatar, whose whole job has been to be a bridge between all nations. Not only does the Avatar cycle rotate between the nations, but the whole institution of sages that supports the Avatar is pan-national. The Air Nomads travel the land, the water and fire nation engage in trade (before the 100 year war). Besides that the 4 nations themselves seem to be in a relatively small area, taking about 2 months to travel from pole to pole with plenty of stops.
So, taking this holistically, with the 4 nations being a small area, who are culturally and economically connected, it's understandable that there wouldn't be the necessary isolation needed for languages to split.
Hope this helps you in your conversation with Mr. Tolkien








