actionman
Emperor Mongoose
No one said that they would be unrecognizable to themselves?
They would be recognizable as Vilani descended people with Vilani descended culture and language.
The continental dialects of Old Norse evolved into the modern Scandinavian languages, but isolated Icelandic changed relatively little. Consider if every group of Old Norse speakers were isolated. Pronunciation will differ, but written language will probably still be intelligible.
Just as an example, a major element of Vilani culture derives from the fact that there's barely any foodstuffs on Vland that can be eaten without processing. How is that affected by centuries of relative isolation on a world where you can just eat stuff that grows? Likewise, they have a circadian rhythm adapted to a much longer day. Does that survive centuries different light levels, day lengths, seasons, etc. People who live on a world where the planet will kill them will have behavioral differences from people who live in a paradise.
I took into account adaptation to different worlds.
So by the time of the Long Night, circadian rhythms will have adapted, etc., if populations didn't live in sealed habitats, since worlds where people can live outside without protective gear aren't that common. The Vilani of the Ziru Sirka would've brought Vilani foods with them and cultivated them, because that's what they would be used to eating and their bodies would be adapted to those foods. The life on other worlds would probably be less edible that life on Vland, with the exception of Terran foods (which would probably make Vilani ill, since they weren't adapted to them).My point is that populations who have been adapted to life on their world for centuries, as part of the Ziru Sirka, will not change all that much ( Edit: because of the Long Night. They would mostly stay as they were when the Long Night occurred, with the adaptations to their planet).
Some Terran cultural diversity is mentioned in DGP and other Solomani-oriented books.
The Ziru Sirka controlled 15,000 worlds, while the Third Imperium only controls 10,000. The Spinward Marches and other frontier regions are the only regions where the Ziru Sirka didn't hold sway. Everything in red on the map below was predominantly Vilani (except for the part in the Vargr Extents), and experienced thousands of years of Vilani cultural imposition and assimilation. 100 minor human races would be 100 worlds out of 15,000. The Solomani Sphere region would be mostly Solomani, because the plagues brought during the Interstellar Wars and the Rule of Man would've destroyed or reduced many Vilani planetary populations.

A better way to say it is the Imperium doesn't force its cultural norms onto planetary populations, but will interfere if a world threatens the Pax Imperia.
Or bucks the Imperial feudal pyramid.
Imperial member world: Imperial nobles should be subject to planetary law while on planet.
Imperium:
