Thoughts about the Hinterlands
by James MishlerIIRC, back when I was working on my massive history and cultural concordance of Mystara, I developed the following basic ideas:
Thratian is not the native tongue of the Hinterlanders; it is a patois of Old Thyatian and the Hinterlander tongue, as well as the other languages of the coast and interior, and is essentially the "common tongue" of the forest region of northern Davania.
The Hinterlanders were M-Welsh and known in their own tongue as Kymraeg; we already had the M-Irish "Arans" in Redstone and the M-Gaels in the southern and northern Isle of Dawn (IMM, the Dunael (M-Celts/M-Dunedain) in the north and the name fails me in the south).
The Hinterlanders were descended from proto-M-Celtic slaves taken by the Nithians from the Isle of Dawn and transported to Davania, just as the Antalian ancestors of the Thyatians, Kerendans, and Hattians were taken from the Northern Reaches.
In fact, IMM, Thyatians were a mix of Antalians and Milenians; Kerendans were a mix of Antalians and proto-Hinterlanders; and Hattians were fairly direct, unmixed descendants from the Antalians.
Most of the Antalian-descended tribes and clans left for the north to settle in what is today Thyatis. The Robrenn and Eusdrians were descended from Anatalian and proto-M-Celtic slaves tsaken by the Nithians to the Savage Coast (who fled north after the fall of Nithia and mixed with the ancestors of the peoples of the Midrealm, then fled south again during various tribal civil wars).
The modern Hinterlanders/Kymraeg are descended primarily from the M-Celts, with a slight mixture of the remaining Antalian clans who remained behind and were assimilated.
Like many Celtic cultures, the Hinterlanders were led by Clan Kings, who might acknowledge Tribal Kings, who might acknowledge Great or High Kings. And there were many, many mid-level kings in between.
IMM the "Land of Four Kings" originally applied only to the Hinterlander territories (the bulk of the western/central forests) and was applied more generally to the whole coast by the Thyatians because, well, that's what colonial powers do. Even if it makes no sense.