I'm guessing that if (as the book suggests) it is traditional, then it is also a (loose) translation.
However, with a quick look at a map of the Americas, Mexico is clearly to the south of a large land mass. The may think of themselves as southerners, or may write for the benefit of their major tourists/visitors, who will be from the North.

However, I make no claims. I just transcribed it as it seems a worthwhile tune. When we sang it in school it was definitely more legato than the sites for ABCs seems to render it!

Cheers

Nigel