It’s more than a little wonky because Blizzard didn’t actually nail down a decent timeline and what we have is not really conducive to cultural development. What I mean by that is currently, the Arathi Empire was founded 2700 years ago, which is when all the disparate human tribes and clans around that general area were assimilated into the general Arathorian culture because Thoradin asked nicely and not for any other reason.
However…
Drustvar quests state that the first Kul Tiran settlers fought the Drust 2600 years ago. Which means that if we use those two dates, you’d need to assume the empire made a colony in Gilneas, that colony developed its own culture, and then became independent and skipped off to found its own colony in… about a century.
This is, of course, very dumb.
Of course if we assume the writers communicate with complex charades and smoke signals while concussed and thus the 2600 number was a mistake, we can probably go with a short while after the introduction of the Light to humanity as a way to judge when Kul Tiras was founded and eventually broke away.
Culturally the biggest differences are religion. The Light barely has a foothold in Kul Tiras, especially since they left the Alliance (though we can assume there’s a resurgence now that they’re rejoined). It has the strongest influence in Drustvar, and you can find the grave of a paladin there, likely because that’s where the witches historically were, and blasting them with holy magic works wonders. In WC3, there are also Kul Tiran paladins with Daelin Proudmoore. But by and large, the Tide Faith holds the greatest sway over the bulk of the nation.
Harvest Witches and Thornspeakers share superficial similarities, but the latter are much stronger than the Harvest Witches were shown to be before they were given a big boost by way of a forcible Emerald Dream connection via affliction. Since Thornspeakers learned from Drust, and Drust are/were Vrykul, we can assume they have the same roots (pun intended), but the Harvest Witches diverged when introduced to Cenarion Druidism, which the Thornspeakers do not use.
To sum up more succinctly, from what I’ve seen from sources like Hearthstone (such as the Witchwood expansion), Gilneas and Drustvar are the closest, culturally and environmentally, with the former just more industrialized.
Accents I have no clue, Blizzard can’t even decide what accents Night Elves have, let alone the maelstrom that is deciphering the dozens of English dialects.