So due to most of my friends living in USA I have 2 accounts on my Switch, a USA one and an European one. I play Splatoon 2 using the USA account, but I couldn't play with my friend in it for some weird reason. Today I went into the game to see the Splatfest results as the USA one has ended already and... it's still Splatfest. Meaning, that instead of using your ACCOUNT'S region, it is using the GAME'S region. So if you bought a physical copy in your local store (like I did) you can only play in your own region's Splatfests. I'd assume same would be true for eShop copy, if you got a game from one region, you can only play in that region's Splatfest as well.