How to change nationalities for several players at once?

I'm trying to create a whole new league and a new nation. I need to change the nationality of many players to a different one. I tried using the editor but it can't do it. In the editor I can select multiple players, I can't do anythign to them while they're all selected. In the editor I can only change their nationality one by one. This would take a very long time.

Is there a file somewhere in which I could edit players nationalities more easily and effectively? Maybe change the nationality of an entire nation to a different one.