This is an ambitious project, and I'd like to congratulate you for the work that must have gone into making this. It's good to see people trying to approximate things that are currently missing from Vanilla DF. Still, I am of mixed feelings about certain parts of the project.
Beware, this is something of a rant.
When fantasy books or games make a direct allegory to a culture from the real world, I admit that it makes me wince. Not only does it seem corney and contrived, but it also drastically oversimplifies a myriad of peoples with different ways of life into fusions that reek of Western Skin-Color based grouping of peoples and cultures. I like the notion of different "cultures" existing within a given race in DF, and this is a nice alternative until Toady codes some method for factions of a race to develop styles of dress, modified ethics, and cultural affinities for certain styles of weapons/armor/materials.
Still, I'd like to see something a little less Euro-Centric. The fact that the "Western" peoples are the ones called "Humans", and everyone else is "different" is a bit sore with me, but additionally the grouping of a multitude of indigenous Asian cultures (Korean, Tibetan, Chinese, Vietnamese, Japanese, to name a few) and indigenous Lapland cultures (Norway, Finland, Sweden, Northern Russia, etc.) into what are essentially skin-color based racial generalizations is more than a little offensive to me.
I'd much prefer to see you create a handful of original cultures, with different ethics, perhaps different tools/technological specialties, and significant differences in clothing style (most of that could be controlled with Clothing Frequency in the Civ Raws) rather than transport Eurocentricised "Earth" cultures into DF fantasyland.
As an aside, calling them "Westerners", "Norse", and "Nippon" is silly, since these names relate to real-world languages, and the first two relate to the Nations' relative positions from Old England. Perhaps you could call them something else, maybe even using the DF Human dictionary to describe some part of their ethics or culture, or something about what sets them appart. Even just calling them the "Sun Children", the "Plainsmen", and the "Winterbloods" or something would alleviate some of the Real-World stigma I'm getting right now... though they're still all thinly veiled allegories.