True, it takes time, and usually war, occupation, and a determined centuries long effort, and even then it usually doesn't work.
Greek culture survived 500 years of Ottoman occupation. Even the Coptic people of Egypt survived. Assyrians still exist today, and they still speak Neo-Aramaic. These nations and cultures survived despite determined efforts from outside forces to assimilate them.
Consider:
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- Some AI Searchbot
Vilani populations at the time of the Long Night became isolated. IMO culture would shift depending on the how much the cessation of interstellar trade would affect people's lives. If a world could maintain itself on its own, I doubt there would be all that much shift. If things became desperate or deadly, then culture would shift in a direction inspired by those conditions.
I suspect that the languages of isolated populations would remain very similar to their languages at the time of isolation over the centuries. There would be no new stimuli to inspire them to change. There would be no loan words from other languages, no new things to come up with words for, etc. While slang and dialect would develop, and pronunciation would change, Vilani-speaking populations which became isolated would still speak a dialect of Vilani.
But they and the other populations of the British diaspora speak English and their cultures are easily recognizable as Anglo-descended cultures. One example is that nations that are descended from primarily British populations wait in lines, while continental Europeans generally don't place much emphasis on that. Something else to consider is that Americans, Australians, and others were not homogenous populations of English people that were isolated for several hundred years. They were mixtures of populations from many nations, of which British populations were the largest part. They were constantly adapting to new conditions and new populations with new languages and customs. In the USA, an important reason why New England and the South are different is that they were settled by different groups of Britons. New England was settled by Midlands Puritans, while the South was settled by people from different parts of the UK, with a large Welsh and Irish contingent. Appalachia was by a mixture of populations from the British Isles as well as by Germans. Another thing is that someone ceases to be considered English as soon as he "loses proof of it in his speech" and doesn't appear to have grown up in the old neighborhood.
Another example of how languages can be slow to change is Pitcairn Island. As late as the 1950's, people of Pitcairn Island spoke English with distinct identifiable regional British accents, because they learned to speak English at home from the original sailors of the HMS Bounty and their descendants. There's also Montserrat in the Carribbean where people still speak English with an Irish accent because of all the Irish people that Cromwell deported there.
But of course languages do shift and change. 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. They're still going to speak a dialect of Vilani that is recognizable as the form of Vilani they were speaking at the time of their isolation. Unless the Long Night destroys their way of life so that they are forced to change drastically, I doubt they'd change all that much.