Since it’s come up in the comments and elsewhere, I thought I’d give a quick rundown of my read on the Valyrian in the world as it exists in A Song of Ice and Fire. It’ll be useful to refer to this map in the discussion to come, since I’m going to be talking mostly about Slaver’s Bay.
High Valyrian was spoken in Valyria for centuries. The Ghiscari Empire was preeminent in the ancient history, and five times they tried to conquer Valyria. Each time they failed, as Valyria had dragons, which they used to repel the invasion. After the last attempt, the Valyrian army wiped the capital of the Ghiscari Empire, Old Ghis, off the face of the planet, and the empire fell, Ghiscari culture being displaced by Valyrian culture. At this time, the Valyrian Freehold took control of Slaver’s Bay, and three formerly small cities became large and rather important: Astapor, Yunkai and Meereen.
Looking at the map, it makes sense to me that the way Valyria interacted with these three cities was by boat. An army could march overland and get to Meereen, then Yunkai, then Astapor, but why bother? A trip by boat is much quicker. Consequently, Astapor is the closest of these three cities to Valyria. (Oh, and if you’re wondering why Daenerys, who, presumably, is coming from the north, goes to Astapor, Yunkai and then Meereen, as opposed to the other way around, it’s because she traveled all the way to Qarth first, and then traveled from there. Qarth is way east and south of Astapor.)
In these cities in Slaver’s Bay, everyone would have spoken Ghiscari, during the ancient wars. When Old Ghis fell, though, a Valyrian ruling class would have had to have been installed, and High Valyrian would have replaced Ghiscari by fiat, and also in practice. When this happens, it generally takes three generations for a language to be lost in a single family. In five or six generations, the Ghiscari language could have been stamped out, if that was a goal of the Valyrians (and it was, I think it’s safe to assume). The old language, though, would have survived in local vocabulary (why lose a word for something that the new language doesn’t even have a word for anyway?), and in the vocabulary of those who weren’t taught the new language explicitly. The result ends up being a Valyrian language grammatically, but with a lot of Ghiscari vocabulary.
Now, all this time, High Valyrian could have been maintained. With the presence of a home base in Valyria and a Valyrian upper class, there would always be motivation to maintain the original language. It seems likely that Valyrians would care about maintaining the language so they could communicate with every part of their vast Freehold. So even as new languages are emerging amongst the lower classes in Slaver’s Bay, High Valyrian would carry on.
The aggravating factor in this history is the mysterious Doom of Valyria, which we don’t know a whole lot about. The Doom was some sort of cataclysmic event that destroyed Valyria and left physical scars all over the region. Not even sailors were go near it now. It’s considered haunted and/or cursed. Linguistically, this is when the umbilical cord was severed for the various outposts of the Valyrian Freehold. I’ll leave the Free Cities out of this discussion for the time being and instead focus on two areas: Slaver’s Bay and Dragonstone.
Dragonstone was founded by the House Targaryen before the Doom of Valyria. It’s located in Blackwater Bay, and is a stone’s throw from King’s Landing (which didn’t have that name at the time). Initially it was established as an outpost to facilitate trade between the Valyrian Freehold and Westeros. Consequently, the Targaryens here would be upper class High Valyrian speakers. After the doom, Aegon I conquered Westeros, and the Targaryen dynasty was established. Naturally, they would have to learn the Common Tongue (it’d just make things simpler), but it doesn’t mean that they’d lose High Valyrian. Valyrian is the tie not only to the old Freehold, but to Essos and the old culture. It would easily have been retained over at least the first two generations. Thereafter, if it was important, it could be maintained through family use and careful instruction. It takes resources to do so, naturally, but they’re royalty; they’ve got resources. So to me it makes sense that High Valyrian is maintained by the Targaryens.
The evolution of the language is difficult to map realistically, since the time depth is greater than the real world analogues George R. R. Martin used. For example, at least 5,000 years are supposed to have passed between the old days of Valyria and the Doom. From 0 CE to today, Latin went from being an everyday spoken language to not existing. In fantasy, though, there’s a bit of wiggle room. I like to think that the rate of change in High Valyrian was accelerated by two factors: (1) contact with other languages; and (2) distance from Valyria.
In the case of Dragonstone, the Targaryens were far from Valyria, but also weren’t really mixing with Common Tongue speakers, per se. They kind of kept to themselves. So rather than change, the language is preserved, while the other varieties of Valyrian evolve past it. Low Valyrian never touched Dragonstone. When it comes to pronunciation, though, Common Tongue pronunciations did end up affecting the Targaryens. This is why older pronunciations of j and v aren’t maintained in the otherwise pristine form of High Valyrian spoken by the Targaryens.
Back to Slaver’s Bay. Although Yunkai is geographically closer to Meereen, I’ve always thought of it as being closer to Astapor culturally. Looking back, I’m not sure how precisely I came to this determination (I admit that). It felt, though, that Yunkish Valyrian and Astapori Valyrian would be closer to each other than either is to Meereenese Valyrian.
Each of the dialects (and I would characterize them as dialects of a kind of “Ghiscari Valyrian”) would be grammatically very similar. They have a common culture, and seem to exist in a kind of symbiotic way, with each city having something the others don’t. Since Meereen is the largest, it likely also has the largest lower class. This is where I saw the most distinct form of the language emerging. This is why it made sense to me that Meereen could support a Valyrian variant that’s quite different in sound from the other two. It’s the same language, but it’s developed its own distinct character.
With Daenerys, she grew up with High Valyrian from Viserys and from the loyalists that helped raised them. In Essos, she would’ve been exposed to a ton of different Valyrian dialects from the Free Cities. This would help her be able to pick up a new one. And, of course, if you look at Astapori Valyrian and compare it to High Valyrian, though there are sound changes, they’re not that drastic. I think it’s plausible that Dany could get the gist of it, even if she can’t speak it. Meereenese, though, is tougher. It’s hard to see a word and tie it to an Astapori Valyrian word, let alone a High Valyrian word.
Regarding comparisons, I likened Meereenese Valyrian to Scots English and Astapori Valyrian to Southern California English. They’re way different, but they’re the same language with some vocabulary items that differ. A couple of commenters have likened the two to Spanish and Portuguese. I simply don’t know if I’d go that far. If I see Portuguese written out, I can kind of get the gist of it, but hearing it? I get nothing. If I studied it a little bit and got used to the sound changes, I mean, maybe, but I’m not sure they’re close enough grammatically. In some ways, Portuguese and Spanish are too close, and in other ways, too far. The pronunciation of Portuguese and Spanish is closer than the pronunciation of Meereenese and Astapori, but the grammar is much further apart. This is why I really think of them as dialects not separate languages.
As for Yunkish, I don’t put it in the middle of the two dialects. Rather, it’s all but identical to Astapori. Truth be told, I haven’t had to do anything specific for Yunkish, but if I did, the variation would be minor.
If I’ve left anything out, leave me a note in the comments and I’ll add it to this explanation. It isn’t as thorough as it could be, but it’s a start. The Valyrian language family is really a fun linguistic experiment, so I wanted to at least give you an idea how I was approaching it. Thanks for reading!
Edit: Some thoughts on New Ghis. New Ghis is an island to the south of Slaver’s Bay:
Regarding New Ghis, where I would start is with the notion that the Ghiscari culture was wiped off the face of the Earth. If we accept that as a truth, we have to accept that they’re speaking some form of Valyrian in New Ghis. New Ghis is pretty far from Astapor, Yunkai and Meereen, so one would have to expect it to be quite different, but how is a question I haven’t dealt with yet. Presumably they can still converse with the cities on the mainland (this happens in Book 5), so it couldn’t have diverged too much. At this point, I think that’s all we can say about New Ghis.