We clearly know that in Othard they don't speak the common Eorzean tongue, considering Hancock is so eager to share his knowledge of Hingan words and their meanings. And he splashes those words into his Eorzean more than is proper for sure.

As far the WoL goes, it's pretty much a given that we have the Echo power to transcend words. If I recall this was played with more in 1.x where the beast tribes didn't speak Eorzean common and your ability to understand them wasn't common.