In America “root” means “barrack” so probably the translation pun you were thinking of wasn’t intended.
Barrack in British English can mean to jeer or boo someone. That's the opposite of root. I've never heard the word barrack used like this in America.
As a verb, it means "cheer" - they are cheering for him.
Plenty of other meanings use as a noun, but that's not how it's used here
Also the somewhat old fashioned phrase “Up the [magpies or other team]” also would have the opposite meaning in the US than it does in Aus. In fact I hear the US meaning more and more, probably due to TV