I think Piotr was about 16 or 17 in Giant Size #1, and is only in his mid 20's at most now. I think the more senior X-Men like Cyclops and Storm are still barely 30. It's a sliding timescale. It's only been like 10-13 years for their whole history(from 1963).
I think you can be a globe/dimension/timehopping superhero and still not be 'Americanized' after 7-8 years. I think most of the X-Men would still have a lot of their cultural heritage at the forefront of their personalities because they are living/working with such a diverse group of peers; there really isn't a 'normal' to conform to(other than being an X-Man).
A character like Piotr suffers more from writers not knowing what to do with him in the narrative, moreso than him being Russian. I mean, not even Claremont really did much with Colossus, in the grand scheme of things. What do you do with a superstrong/tough guy who is not the team leader, or the main love story, or even the most powerful character on the roster? Most of his stories have been tragedies because that's the only thing left for an invulnerable man: seeing those around him fall, causing him sadness.
He has been written into a corner, but it's hard to see a way out for him as a character within the team dynamic. Only like, a solo book, or at least giving him a spinoff team to lead, could really move him forward narrative wise.