Major Spoilers for all Expansions - Some general things I didn't understand

[Cilia]

Needless to say, ARR changed this somewhat where this whole thing was kind of brushed over with minimal explanation and everyone just speaks the same language (although the Ascians were initially depicted as speaking their own outright alien language, and this even popped up in ShB and EW from time to time with Ancients also speaking alien-like gibberish, and yet other times the Ancients could be easily understood, especially when the player goes to Elpis, so it still was rather confusing.).

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.

(Doesn't explain everyone else after the plot point is kind of dropped after 2.5... but, plot convenience.)

[Cleretic]

The 'Echo as universal translator' thing holds well enough for long enough that it feels like they just sort of expect you to not be thinking about when we finally hit the times it doesn't--which for my money actually isn't any of the times we're talking to people who shouldn't know how to speak Eorzean (because again, universal translator), but in Stormblood when we head to the Far East and Alisaie and Lyse can communicate to everyone just fine. Alisaie I could at least understand being learned enough to have a second language, but Lyse... no.

I mostly accept it by virtue of recognizing that the storytelling would just be worse if there was a language barrier. We wouldn't gain anything from Lyse constantly asking 'so what did they say' all the way through Yanxia and the Azim Steppe, so she doesn't.

[Cilia]

More to my point there's the Warriors of Darkness and Norvrandt, where despite coming from a parallel world with vastly different history they still speak the same language. There's ways to explain it on both ends (the Ascians taught them Eorzean / the time differential gave the called Scions enough time to learn the language), but it still rankles a bit.

I do acknowledge and accept it as a necessary storytelling contrivance, at any rate.

[Zero-ELEC]

I mean, Y'shtola did acknowledge during the our time at Troia that the voidsent basically speak a language close enough to Eorzean to be understood but not exactly the same language.

[Cleretic]

I mean, Y'shtola did acknowledge during the our time at Troia that the voidsent basically speak a language close enough to Eorzean to be understood but not exactly the same language.

And this is a basic rule of multiversal storytelling: No matter how far the world you've traveled to diverges from the timeline you know, everyone still speaks English (or whatever language the story is written in).

Realistically, the fact Y'shtola can have conversations with Zero and Runar is more believable than the fact that she can also have conversations with Squall and Zidane anyway.

[Lady_Silvermoon]

4. He was probably so faded at that point there wouldn't be enough left to bring back. And remember the power to yoink people out of the lifestream was a temporary one Hydaelyn gave us probably hoping we'd use it on Emet-Selch as an apology for her using him to kill people for 12k years. But he did his duty and peaced out back to the aetherial sea.