I have a headcanon that all the clans have slight variations in dialect. Most of the time cats don't bother learning another clans dialect but the leaders and medicine cats have to know the differences. They usually start learning as deputy or after starclan accepts them as a medicine cat. The difference between riverclan and shadow clan is bigger than riverclan and thunderclan and skyclan is very different from the other 4 clans. They kinda have accents with how they talk like the other clans.