Languages are simply not discussed much in the game, it's probably just left out in the writing, similar to how we don't see evidence of time-zone differences.
It could be explained by the existence of a common international language, perhaps Allagan, which is likely the root of most current languages.
If it's Allagan, it'd make sense if the Garleans picked that up as their national language as well, makes it easy to study relics after all.