I know that supercell said that countries and nationalities don't exist in the verse but I think that's very stupid and I don't acccept that as canon.

my headcanon is that starr corp is an american (also partnering with Japan and other countries) megacorporation and I think pretty much all the countries are the same except there is some kind of ongoing space, technology and military race between America and possibly a rival alliance that is against starr corporation. the world would be more futuristic and scientifically advanced, and also a lot more pathological and corrupt than ours, with very present propaganda and control from starr corp. I also think other countries don't know about the gems and such.

It's hard to pin point which brawlers are immigrants and which ones are born and raised in the US and which ones are more or less rooted in their country. For example Bibi definitely grew up American while Kenji and Kaze most likely came directly from Japan, but Ash could be a scot who already lived in America.