Players are able to play for a different national team, like Diego costa played for Spain in 2014, if they have citizenship in the other country. But I’m pretty sure they can’t switch after playing competitive matches for their “original” country’s national team. So mbappe has obviously already played for France, I don’t think he can switch at this point. He may get Spanish citizenship in the next five years playing in Madrid, but he’s tied to France at this point.
Players are able to play for a different national team, like Diego costa played for Spain in 2014, if they have citizenship in the other country. But I’m pretty sure they can’t switch after playing competitive matches for their “original” country’s national team. So mbappe has obviously already played for France, I don’t think he can switch at this point. He may get Spanish citizenship in the next five years playing in Madrid, but he’s tied to France at this point.