Oh I don’t know, it’s kinda like when you go visit the Vatican you don’t wear shorts and short sleeves. With a game, either wear the colors of the teams playing or go neutral if you aren’t aligned, but wearing another team’s jersey is kinda douche in that it says the person wants to be taken seriously as a soccer fan because they like a particular club/country enough to have the kit.
I can’t think of a single college or pro football game, or MLB game I’ve ever been to where fans are wearing third-party jerseys.