I know they speak Slavonic, I mentioned how I would be unsurprised if the covenfolk spoke Slavonic rather than Greek.
But my idea that Greek would be the covenant language is not un-historic. It depends on where you put Coeris. If it's in the Southern Carpathians, perhaps on the Danube river valley side, then Greek is a plausible option. I provided sources last time we talked about this; in fact,this is pretty much the same debate we had last time.
The part you acknowledge and then discard is the fact that in that part of the world Greek was the language of the educated.
Who runs the covenant? The educated people, who'll likely want a language they all have in common, which, if they were educated in the largest urban center nearby, which would be Constantinople, the dominant language would be...Greek! Who would be teaching initial reading and writing to apprentices? Probably the same specialists.
If the covenant wants easy trade with the biggest source of quality materials in the area, which is again-- Constantinople, they're going to want to speak Greek. That trade is going to be a lot easier than trying to get to any of the other major urban centers in the area, with a quick hike down to the Danube, then out to the Black Sea compared to long hikes over mountainous terrain.
Now, if you've got Coeris further north, more towards Germany and (what would become) Poland, or more towards central Hungary, then yes, Greek makes little sense. However, the fact that HoH:TL states that Coeris is built on the Gate of Eurydice almost implies that it is in an area influenced by or involved with Greek culture. That's another point to consider. Why would a site in a more germanic area reference a hellenic myth?
Not only that, but HoH:TL mentions how the Byzantines invaded Hungary and Bulgaria 20 times in the 12C...so while Constantinople may never have "occupied" Wallachia, they obviously had some considerable influence in the area.
Personally, you pick what you want, but I don't think it's fair to say that the option of Greek is historically inaccurate. There's plenty of history that supports such a choice.
-Ben.