Honestly, language is usually the one thing developpers dont tend to develop and make too realistic as it would makes conversation with anything non local quite complicated.

While they all speak the same common tongue, this is obviously for convenience.
Closest nation could have language similarities, like French and Italian. But the further you'd go the more different the language would become.


So yes it is extremely fair to assume that in a more realistic scenario Garlemald would have their own language.

Same for the Alagan empire, how convenient that every time we encounter something from that old time we can understand them whereas in reality, depending on the language you only have to go back 200 years to get an almost entirely different language. (For instance Japanese is a language that changed extremely fast)

Language change the faster when the community lives in a time where education is poor. (basically common speech is dialect speed and since there's no authority to say what's right or wrong, changes occur very quick). So After the Alagan empire crashed, you should have technically had a quick language evolution. Fast forwards a few millenia to our present time, it should be a totally different language with some recogisable roots for some basic words.