Any demon conjured by his true name will know the tongue of his master, sort of automatically. Built into the spells/magic/I have no idea, just is.
Now different masters can summon in different tongues…
So, those of us who’ve been around the Astlanian block a few times have picked up multiple languages, in many cases.
Plus there is a common tongue for merchants and academics, and pretty much any region that was part of the Vargosite Empire speaks it, although maybe not exclusively. The various Etonian religions have also exported it to other continents that weren’t part of the Vargosite Empire. So if the Etonians are there, someone will speak it.
It’s what they speak in the Council States, it’s what Tom speaks.
You might call it Vargosian, except that it’s derived from Anna-An the language of the Anilords, which is in turn derived from ancient Anian (of the An empire) Not sure where it came from before then. Modern Vargosian isn’t actually that similar to Anian anymore, other than some root words, but I don’t think I’ve heard anyone speak Anian (other then a few demons) in close to two thousand years.