From what I've seen so far there seems to be three different kinds of civilians that walk around towns in TES II:

-Middle Ages Europeans (Daggerfall)
-Africans (cities in Hammerfell)
-mystery hairy guys in heavier clothing and kilts (in the eastern cities and towns)

Are these kilted peoples culturally Nords? Colovians?

My best guess is Reachmen, though I'm not sure if the cities they're in are far enough east. Or maybe the 'horse-people' of the Bjoulsae I've seen mentioned in a book somewhere.

How could we know from a single sprite?

Well, you can see the "kilted" type of townspeople on Balfiera, which I would think means they aren't Reachmen or the Bjoulsae tribesmen. Reachmen are a group of magically inclined barbaric people that are located at high rock's border with Skyrim, right?
I think that "Breton" probably isn't a single, homogeneous culture. There are probably a lot of distinct cultures in High Rock, so we see people dressed in middle-age type clothing, kilts, and a lot else besides. Looking different than the other bretons doesn't mean they must be Nords or something.