Sure beats "here we go, here we go..."
I was raised on English terrace chants so I've no idea what's going on here. (I also don't speak Spanish) I'm assuming it's before the gates open. Whatever it is, it sure got me pumped up for a match.
The music is that of "Matador" by Argentine band Los Fabulosos Cadillacs. Speaking of Argentina, you haven't heard singing and felt stadium bouncing until you've been to La Bombonera for a Boca Jrs. match. Thanks for the cool UNAM clip.