I don't really see the difference between nationalistic and regionalistic sentiment.
Then perhaps you shouldn't really make broad comparisons/analogies of things that aren't quite the same.
Club football has little to no nationalistic sentiment as a whole, despite people who adhere to that ideology (hardcore alt-rights, neo-nazis and even others like anarchists) infiltrating plenty of "ultra supporters" sections and causing issues from time to time. What people gotta remember is that despite those problems,
which we should 100% curbstomp as best we can, these alt-rights/neo-nazis represent a tiny faction of most (if not all) club supporters/fans.
Regionalistic sentiment is, usually, between people of the same country. And is usually (in the context of football) more in the vein of "if I lose, I hope they also lose" rather than "fuck those guys and everything they ever stood for, just because they were born 20km away from here", since in everything else those same people get along completely fine. To say this, which is the most commom sentiment in club football (even when talking about teams from different countries) is the same as nationalism is silly.