Quick relevant nation blurbs:
Azlant: Azlant no longer exists, its destruction by a summoned meteor from on high over six millennia ago was a cataclysmic event that basically ended civilization for a while, plunged the world into darkness for an extended period, and a variety of other miseries. It was an impossible civilization of godlike magic and power and sophistication, a (now ruined) continent unto itself, and a modern source of scholarly curiosity to obsession. I mention it largely for that the Taldan ethnicity is the result of Azlanti refugees blending with whatever the local nomads in Taldor were (some Keleshite-ish people or something close enough). Taldor likes to view itself as an inheritor of Azlant (but so do several other nations), and being able to show or claim to Azlanti heritage gets some regard, the main cue for such being purple eyes that show up with varying frequency.
Cheliax: Cheliax is actually nowhere near Taldor, geographically speaking, it's the nation on the far west coast of Avistan, the continent they are both on, with several nations inbetween. They are, besides Taldor, Andoran, Absalom and Qadira, one of the big naval powers of the Inner Sea, and a rival to them in that sense, but it's not like they might invade some day. Country named for its main ethnicity, the Chelish (who by contrast to the skew closer to bronze skin and brown hair of the Taldans, are more pale and sharper featured), who are another descended from Azlanti refugees marrying into the local ethnicity of that particular area. Mentioned largely for that it used to be a vassal state of the great Taldan empire for a good long while before declaring independence some 600 some odd years ago when Taldor was too busy with a devastating war with Qadira to do anything about it. They would go on to gobble up most of the states Taldor founded via either diplomacy or conquest, and even grab the patronage of the church of Aroden, god of humanity, for themselves, becoming the newest great empire of Avistan. They're kind of.. the Holy Roman Empire to Taldor's Byzantium if you want to do rough real world mapping that is not quite accurate. Their entire society was gearing up for the prophesied return of Aroden to the world, who would lead humanity to a new Azlanti style golden age (doing so from Cheliax, of course). Then 100 years ago or so, instead of returning, Aroden died (means unknown), breaking prophecy in general across the world, calamities both metaphysical, physical and political ensuing. In Cheliax, it lead to the empire falling into brutal mutli sided civil war that was ultimately won by the Hell tainted, pacting and worshipping house of Thrune, and the spreading influence of Hell across their lands was the price of the end of chaos and massacre. A variety of vassal nations were not particularly thrilled at the idea of Asmodeus as a state religion, or devils walking the land, and rebellions ensued, vastly reducing the Chelish kingdom in size from a former glory it schemes to reclaim to this day. The two particularly relevant nations that broke off are on Taldor's borders, which leads us to...
Galt: Galt is essentially one neverending Reign of Terror, French Revolution styles, with successive revolutionary governments consuming themselves, backed by a fanatic, frenzied mob of a population. It was Galt where democratic philosophies to rebel against Cheliax with were created, got wildly out of control, and lead to the deaths of some of those very philosophers. The country lurches from one riot to the next, ready to turn on some new scapegoat at any moment, eager to put to use guillotines that trap the souls of those they behead within them. They're to the north of Taldor, and the nation debates whether to try and invade Galt to pacify it or not. A more successful version of Galt is instead...
Andoran: Okay, let me get this out of the way. The more I think about it, the more I dislike Andoran. I put it this way to someone. There's a valid school of thought that complains that fantasy idealizes monarchies, and sure, that is a thing. But Andoran is like some kind of ludicrous overreaction to that. Andoran is basically a whoa idealized version of post Revolutionary War America (even from the colour and pattern of their uniforms), without any of the bad parts. They are the all triumphant designated good guys (whereas by contrast, the lawful good nation of Lastwall is being slowly ground down to extinction without anything to do about it but make the enemy pay in blood for every inch of land they lose, as far as having actual problems and pathos) Hell, the president equivalent (Supreme Elect or whatever) is a Paladin who seems to just breeze through most problems. And why not, Andoran even has direct patronage from the celestials. The only handwave to this society not being so perfectly perfect in every way that Mary Poppins herself would be all "daaaaammn!" is that they treat the fey and their forests like garbage, and even that is somehow only because the government's hands are tied as far as this one eeeeviiil lumber consortium, somehow.
Okay, with all that out of the way. Andoran. Inspired by the philosophers of Galt but of course much more successful (because of course they would be, when does Andoran ever fai- I'm going to stop now, I promise), the People's Rebellion of Andoran threw of the shackles of their infernal masters, forming a free and egalitarian society, so long as you aren't a fey living in Andoran's half of the great Verduran Forest. Defended by a powerful navy, and the heroic order of the Eagle Knights, Andoran seeks to spread their ideals throughout Avistan both overtly and covertly, while trying to end slavery as an institution and making the GM puke in his mouth a little bit (okay, I'll stop now, for realises). They are on Taldor's western border, and other than a duke getting it into his head to try and "pacify" Andoran some years back and his expeditionary force getting wiped out, don't interact /too/ much with Taldor beyond low level intrigues for influence in places like the huge independent trading city state of Absalom, the usual efforts for Andoran to spread their ideology, and both sides keeping a heavy watch on the shared border. That Taldor has a successful centuries old agreement with the Druids and fey of their half of the Verduran that has allowed them to husband it for resources, whereas Andoran is in an expansionist, resource use intensive mode may someday be a flashpoint of conflict.
Don't mind my hating, if you want some kind of positive ties to Andoran, that is actually okay. It will not make me go out of my way to murder circus you.
Qadira: A sort of fusion of the Ottoman Empire, the Caliphate of Baghdad and.. let's just break down and say it is basically fantasy Arabia as my quickest way of summing it up, the autonomous Satrapy of Qadira (technically a vassal of the greater and far off Padishah Empire of Kelesh, but able to basically /mostly/ decide its own policy, and sometimes conquer and rule whole other nations) has been Taldor's most determined enemy for thousands of years, each nation periodically trying to conquer the hell out of the other on land and sea in wars that sometimes last centuries. Current the conflict is just low grade raiding and a cold war otherwise, but no one expects that to last without some resolution, whether a new war or some binding treaty. They worship a more militant and mildly heretical take on Sarenae down there mostly, and that's part of why the worship of Sarenae is banned in Taldor. One of the really big slave owning nations of the Inner Sea (the others being Cheliax and Katapesh. Slavery is certainly legal in other places, Taldor included, but isn't some crucial part of the economy, with vast slave labour forces. Taldor after all has something of a vast semi indoctrinated underclass. Who needs slave labour that badly by comparison?)
They're the southern border of Taldor.
The Padishah Empire of Kelesh: Origin point of the Keleshite ethnicity, there's actually not too much detail on these fellows, other than that they rule a vast empire of many satrapies across the continent of Casmaron, and while including the remnants of fantasy Sumeria, they are basically fantasy Arabia /even more hardcore/. While Taldor's eastern border is not really inhabited to any vast degree, it is technically under the loose control of the Padishah Empire directly (as opposed to being held by a largely autonomous satrapy), and would be an access point for them to invade directly, if they ever wanted to. Part of why there were relatively recent efforts to heavily fortify it.