Riffing off a couple of posts below: the original Encyclopedia Eorzea notes that on the Steppe, most Xaela still speak their ancestral language. We get hints of it with the occaasional Mongolian words and names in Stormblood, but at no point is there any indication of an actual language barrier. Obviously the Warrior of Light has the Echo, and you'd expect the non-Qesteri Xaela in Reunion to speak the common tongue, but there's several Mol children are able to converse with Lyse with no trouble, and Gosetsu certainly makes his opinions of Dotharl customs clear. How, exactly, does that work?

On a related question: does the Echo transmit accents? Would a Steppe-born WoL have an accent when speaking in Eorzean?