I’d already heard the explanation that Common is the pidgin (or whatever the term is) of the different languages known by slaves, as they move from city to city.
And each city has its own language. With a variety of writing systems.
But I just learned that each trade house has its own language. And nobles have their own languages, too; sometimes, different noble houses in the same city have their own district languages!
That’s a lot of languages!
Consider, though, that Europe has 250 languages native to it, and Australia had about the same number at the time of colonization. While we talk about French, Spanish, Italian, German, etc., those are historically areas with several different languages. They might be somewhat mutually intelligible (Asticles and Vordoun might be very similar to Tyrian, for example).
For my part, I just went with “almost everyone speaks Common, with most cities having a unique dialect that includes words from their previous languages, especially in proper names.” So, everyone calls “rice” “rice”, because if you’re selling it to another city, you need a common language. But the Drajan word Maize (for example) gets used all over the Tablelands, and Draj itself has a lot of nuance for different kinds of Maize.
And some of these could be codes, like secret languages for all the noble families of Tyr that are their own unique dialects, or coded merchant written languages.
I’m honestly not sure what would be ‘realistic’.
From one perspective, the Tyr Region is tiny. Not comparable to Australia or Europe … at ~100,000 square miles it’s more like the UK or New Zealand. And all the rulers share a common origin. From that perspective, there really should only be one language, at least in the city-states.
From another perspective, literacy is super rare, so language change will probably be rapid. If trade and exchange of population between the city states isn’t enough to stabilize the language, each city-state might develop its own.
From yet another perspective, especially if you believe the published population figures for the city states, being ruled by immortals ought to suppress language drift. Nobles and templars will probably consciously imitate how the sorcerer-monarch speaks, which might nearly block natural language change. I am not sure if city-states of 8,500-40,000 are large enough to support a lower class dialect/language diverging on its own from the upper class language. (Ur Draxa does canonically have this, Low Draxan/High Draxan, but this seems intentionally enforced not a natural development.)
You can argue either way, but given the size of the setting and the immortal rulers, essentially universal Common feels right to me. There should absolutely be secret languages, and relics of other languages, etc. - but I don’t think anyone raised in one of the seven city-states wouldn’t speak Common. (Though slaves captured in the wilderness and recently brought to a city-state might not, especially if they’re e.g. thri-kreen.)
Places outside the Tyr Region - Ur Draxa, Last Sea, Deadlands, Kreen Empire, etc. - of course would have their own languages.
Having Common though, doesn’t preclude having other languages - the idea I’ve always heard is that nearly everyone speaks and understands Common, but free citizens are less likely to speak it and nobles deteste speaking the “slave tongue”, so a merchant might reasonably speak several “human” languages (Common, Nibenese, House Shom’s tongue, bits of Balican from spending so much time there, etc.) in addition to a few racial ones (elven and gith).
I can see that… except that to me it makes way more sense for Common to have initially been the language of the aristocracy, not of the slaves. Because the Champions presumably had a common language way back when, and if the templars & nobles imitate the SKs (as I would expect), the upper classes of the cities would all still be speaking the same language they were upon founding ~2000 years ago.
It’s the lower classes, not in direct contact with the SKs, that might diverge. But I’m not sure if city-states of 8500-40000 people are large enough for upper vs lower class language divergence or not.
Yeah, but all the Champions/SMs came from several different cultures and spoke several different languages (obviously they all managed to speak with Rajaat and each other) - the languages of the Green Age and their relationship to current languages is discussed starting on pg 16 of Faces of the Dead Lands.
Essentially, the pitch is that there’s 2-3 ancient languages that formed the main basis of Common and moat of the city-state languages, due to how the city-states were settled/conquered.
The reason I go with “Common is universal” is time. Thousands of years, in a smallish area, you’re going to develop some means of communication with each other, and languages will meld and mash a bit. Most languages keep their proper nouns and words that Common doesn’t commonly (pun intended) need, but, over time, the language just becomes a mess of different influences with an unintelligible grammar.
I don’t know where I came up with this, he says in English. 
Yeah, I pretty much agree, I was just thinking that the melding/mashing process would have already occurred during the 1500 years of the Cleansing Wars. So while the Champions had different native languages, their & their armies’ everyday language might have already developed into Common before they claimed city-states.
Sure, maybe. Keep in mind though that - at least in the Athas.org materials - the Cleansing Armies were often on the move, rarely interacted, and essentially existed as massive mobile nations. And, when they did interact, it was at a high level and often through magic, so the common folk in the armies probably had little chance to interact with each other.
Now, there’s definitely exceptions to this - like when Keltis and his successful army were shacked up with Sielba in/outside Yaramuke. That lasted for quite a while - probably long enough for the Yaramukite and Kurnan languages to be EXTREMELY similar, but then maybe Keltis demanded that his people purge themselves of Yaramuke’s taint after he and the Sielba broke up, dragging Kurnan back to its ancestral linguistic roots and leaving very few commonalities.
TLDR: Yeah, there’s a lot of room for pretty much whatever you want, even if you accept everything written about Athasian languages as true.