Common (Taldane) is only common in the inner sea region. If you go to Tian Xia there is a different common (it's a dialect of Tien). The same could be seen to be true in the 2/3 of Garund that we know nothing about, and in the Kelish Empire, Vudra and Arcadia.
And even if someone speaks common, does that mean they prefer to use it? No. Can it (and would it likely) be seen as some degree of insult to the culture to be forced too? Yes. Then there are dialects, something the game seems to utterly ignore outside of the single spell Cultural Adaptation.
