Generally it's assumed that places like Doma and Kugane speak japanese. I'm sure the devs mentioned something like this at some stage?, and Garlemald is the only known region (the others we know of are in eorzea so they speak the common tongue) that has a different language for their states motto and almost everything named by garleans is in latin so that is a pretty good case to be made. The question is does everyone know the common tongue and just use their main language amongst themselves or has the whole language thing been largely retconned or ignored for convenience?