Actually, my view is extremely influenced
by history prior to 1950. You cite Nazi Germany, but Nazi Germany was actually a
new occurrence. It wasn't from tradition. Hitler was a modern politician, and fascism a new movement. It lends credence to my point, it doesn't take away. The culture that replaced the Nazis, too, was not representative of the culture prior to the Nazi government.
Languages have died prior to 1950. Internationalization comes for all.
Even the Catholic Ireland you cite is a relatively new concept, part of the internationalization of the region. Christianity has a rich history there, but it's only a relatively recent happening that they cared about Papal supremacy; it wasn't a great deal of generations ago that the Church of Ireland was by far the largest, and prior to that, papal supremacy never got that much of a foothold in the region. Presently? 69% of people in the Republic of Ireland believe in God. It's a plurality, but the trend is still down.
Cultures die; any view other than this is ahistorical. Prior to 1950, much of the dialects of French within North America were dead, or close enough that it didn't matter. Quebec is left, but Quebec is considered an oddity. There are a few dialects that are hanging on by a single speaker, but is that really a living culture? Those will be gone in a generation.
Wales was oppressed similarly to Ireland, and Welsh is on the path to dying, too; reactionary pushback won't save it. Try and save the language all you want; it will die. Scots will die, too, despite the pride stereotypical to the region. It's basically already dead; its Wikipedia was written in a fake pidgin of Scots for years, and it took years for anyone to notice.
The failing of civilizations and internal slaughter is inevitable. I agree with that. There's no avoiding it. I think it's funny when people try to avoid it. They will lose, inevitably. My position is without regard for the quality of culture, without concern for what culture is going into, but rather, with full enthusiasm over seeing things turn over, and the reactionary, exclusionary attitudes that come when cultures are near death.
A subculture that will survive for another century isn't filled with people trying to keep it small, it wins via numerical superiority. There's a reason that Latin is the basis of most language in most continents. Californication, in a century, may be known as a Mandarin Shift, or Hindufication. California isn't eternal, but tiny cultures being subsumed by larger ones is.
Certain conservative cultures will stay around, at least for a while. It's likely they'll get bigger, even. Geographically-insular ones rooted in exclusion rather than proselytizing won't, though.