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

[Brightamethyst]

As part of that, the Echo doesn't 'tell' you about telegraphs and AoE fields.... as the devil is always, in the detail. We have ability telegraphs and AoE fields on screen for pretty much everything, even for things that have no souls/aether like boulders and machina which if it really was the Echo telling you that, it would only be able to work with something that has a soul. As inanimate objects/non living entities with no aether/souls also give ability telepgraphs/AoEs, therefore it's not the Echo showing that. It's just a gameplay mechanic with no bearing on the story. The whole game visual interface is just for gameplay purposes, nothing more.

Yeah. While personally I like the "red circles and cast bars are the Echo letting you see 5 seconds into the future" headcanon, it's just that. Headcanon. There's nothing in game that indicates it's an actual Echo power. It's much more likely that it's just one of the many mechanics that exist purely for gameplay reasons, lore need not apply (aetherytes in Elpis, Eorzean gil as a multiversal currency, etc.)

[Eisi]

it's just one of the many mechanics that exist purely for gameplay reasons, lore need not apply (aetherytes in Elpis, Eorzean gil as a multiversal currency, etc.)

1. Aetherytes in Elpis I 100% take as proof that they used Aetherytes in the Ancient world, along with the one in dream Amaurot. Why else would they get their own special model? Could have just as easily made something that's not an aetheryte.

2. Gil as multiversal currency I assume doesn't refer to the First where they went into detail on why Gil is the accepted currency there, which is actually an indicator that they take the lore more seriously than just having stuff exist purely for gameplay reasons (much like the new custom delivery line where spontaneously flying creatures [mounts] are researched). Instead you probably mean Ultima Thule, which only has been given shape through the sacrifice of Thancred, which made it breathable, traversible, tangible, communicable and I would assume also allowed for trade and stuff. Also we have the likely original Omicron currency as a counter as well: Omnicoin.

By the way, they obviously use it for gameplay reasons first and the lore follows that, rather than making the world reflect preexisting lore. It's still lore though.

[Brightamethyst]

2. Gil as multiversal currency I assume doesn't refer to the First where they went into detail on why Gil is the accepted currency there, which is actually an indicator that they take the lore more seriously than just having stuff exist purely for gameplay reasons (much like the new custom delivery line where spontaneously flying creatures [mounts] are researched). Instead you probably mean Ultima Thule, which only has been given shape through the sacrifice of Thancred, which made it breathable, traversible, tangible, communicable and I would assume also allowed for trade and stuff. Also we have the likely original Omicron currency as a counter as well: Omnicoin.

I was talking in a more general sense about the fact that any single currency is accepted absolutely everywhere. From Gradianan merchants, to Vanu Vanu warriors, to the Steppe clans, to 4000-year-old Allagan nodes in Azys La, to Ancients in time before time Elips, everyone accepts and pays out Eorzean gil.

In major trade cities like Kugane and Radz-at-Han (which we know have their own currencies)? Sure, there are probably money changers everywhere and those merchants probably accept loads of different currencies. But everyone else? When a Yanxian peasant gives you a quest to weed his fields, is he really paying you in gil, or is he paying you in local coin and the game just automatically converts it to gil for the sake of convenience?

[Eisi]

I was talking in a more general sense about the fact that any single currency is accepted absolutely everywhere. From Gradianan merchants, to Vanu Vanu warriors, to the Steppe clans, to 4000-year-old Allagan nodes in Azys La, to Ancients in time before time Elips, everyone accepts and pays out Eorzean gil.

In major trade cities like Kugane and Radz-at-Han (which we know have their own currencies)? Sure, there are probably money changers everywhere and those merchants probably accept loads of different currencies. But everyone else? When a Yanxian peasant gives you a quest to weed his fields, is he really paying you in gil, or is he paying you in local coin and the game just automatically converts it to gil for the sake of convenience?

What about language, does everybody in the world also speak the same language, except for some very rare cases? Do we have to assume that for the player's convenience the process of translation between different NPCs is shortened so it just appears that they understand each other?

I'd rather assume that unless stated otherwise, everybody speaks the same language. Same goes for Gil, it's a universal currency because it is. Just like there exists an unlikely explanation on the First as to why they use Gil, so does such an explanation exist for every other example you mention.

[Enkidoh]

What about language, does everybody in the world also speak the same language, except for some very rare cases? Do we have to assume that for the player's convenience the process of translation between different NPCs is shortened so it just appears that they understand each other?

I'd rather assume that unless stated otherwise, everybody speaks the same language.

It's been somewhat ignored for simplicity's sake here in ARR, but 1.0 explained this in better detail, that language is not universal on Hydaelyn, although the citystates of Eorzea all shared the same language, the beast tribes did not, and therefore other lands like Garlemald and the Far East would also have their own languages too (and it was actually mused by certain NPCs in 1.0 to be one of the reasons there was so much animosity between the citystates and the beastmen that was leading to Primal summonings, it was simple inability to effectively communicate between societies that was the cause of much tension, which, unspurprisingly, was being exploited behind the scenes by the Ascians). Even the moogles had their own unintelligible language - the fact that the player's character could understand easily what moogles were saying was the first indication something very strange was going on in 1.0's Gridanian storyline.

The Echo was therefore used as a convinient tool to both make use of this for dramatic tension and also neatly sidestep gameplay issues where it was easy to just have every NPC speak the same language from your perspective with a ready explanation as to why that was possible (and even that was not universal, as it was clear that not every Echo user actually had the ability of xenoglossy - there was a funny scene for instance in 1.0 where sylphs invaded the Waking Sands and not even Minfilia could understand what they were saying - she had to literally get the player's character to translate Sylphic for her).

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.).

[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.)