It's actually a matter of the Echo working as an auto-translator and probably "common" becoming more widespread.
What we hear isn't always what's being spoken; back during 2.5 we're out getting some aether readings with Moenbryda and get ambushed by Nabriales, but she can't understand what he's saying because she doesn't have the Echo. This actually functions not by automatically translating anything being spoken but by connecting to the soul of the speaker, hence why we can't understand Alpha until he gets one in the denouement of the Omega storyline.
We also know the Hrothgar have their own language, as per their blurb on the Shadowbringers site; they only recently learned to speak "common," and the language barrier was a big part of the reason they weren't seen in the Eorzean city-states until that point.
As for why everyone, except for Lyna and Erenville, speak with English accents despite being separated by thousands upon thousands of miles and years of history? Well the only Watsonian answer I can come up with is the Ascians did it. :rolleyes:
(Doesn't explain everyone else after the plot point is kind of dropped after 2.5... but, plot convenience.)
