All of these peoples speak Yhing Hir predominantly I would expect.
What of the Erdukeen? Are they of Yhing Hir stock? Mixed stock I would think. They speak Erdukeen which does not use the Khitani alphabet like Yhing Hir.
What of the Skohir? What are their origins? They speak Skohir, and their language is not listed under the Linguistics skill, but is under nationality - Hinterlands. Is the language Khitani derived, or different?
These are Hinterland people, but I'm not quite sure about them. Different languages, different people.
Erdukeen are the descendents of followers of the Sword of the heavens who relocated there after his fall, thus they are ethnically and linguistically distinct from the rest of the Hinterlands. That isn't to say that there was no interbreeding, but I suspect it was minimal. Because of this, val'Virdan are probably overly represented in their population relatively speaking to most.
Skohir are descended from what are essentially vikings. They appear to predate the arrival of the gods of man on Arcanis, with some motherland off the map somewhere. They generally do not have shaman, but instead priests of the GoM, primarily Yarris. I suspect we will be learning more about them in the future.
You mention one race of man, but don't ask about them- the Pengik. They also appear to predate the arrival of the GoM, and unlike most humans appear elementally attuned so there are interesting guesses about their origins.
What I always find myself wondering about with the Yhing Hir and other tribes of the Hinterlands is, what does the population of Vals look like in their population? Obviously there are lots of val'Virdan in Erdukeen. Probably no vals in the Pengik. Are there val Skohir? Yhing Hir? How does ethnic val work with the "color coding" of the val that all the val of a family look similiar? Does a val'Assante Yhing Hir (if they exist) look like a Coryani val'Assante? It's important for character concept continuity since I would look oddly at someone playing a val Yhing Hir right now. Since they Yhing Hir are descended from the Khitani, are there any Uls running around in their population? It seems like we've had enough contact with them that should have been noticed already if there are. But if there aren't, why aren't there?