Hmm.

There are characters who clearly have accents in Eorzea, which could suggest that they speak another language (for example, the horde of phurble-puff-puff women from the brickman event speak with French accents).

There are mentions of several 'ancient' or 'old' languages in game, which have been replaced by the Eorzean tongue, including a Padjali language of all things. I am confused why Padjali would have a different language if they were just hyuran children transformed, but I digress.