I've thought about this and I don't buy it. Dorne is ethnically Rhoynish, but, with the possible exception of the Orphans of the Greenblood, they speak the Common Tongue as an everyday language at home in Dorne. (Otherwise Areo, Arys, Myrcella et. al would have to be remarkably quick linguistic studies, or everyone…

I thought Doran was fat in the books though? Was he not and I just always assumed that because he's gouty?

Is anybody else bothered by the geographical and lexical sloppiness of the TV series compared to the book? I'd be at peace if I sensed deliberate narrative choices, but I really think there's just a lot of laziness and sloppy casting.