See, that's actually what I'm trying to figure out.
Take Dragon Quest IX, for example. The DQ9 cart at my store is the exact same as the DQ9 cart up in Canada. However, the packaging is different. Up in Canada, there's a big chunk of the front of the box advertising that the game plays in French (at least according to a forumer I know). The DQ9 here has an iiiitty bitty piece on the back saying "Oh yeah, this game plays in English, French, and Spanish, btw. Please don't boycott us for being unamericanz!"
And take, for another example, Disgaea DS. I've got a Canadian copy, and my brother has a U.S. copy. The only difference is the packaging and inserts. Notably, my copy has 2 separate manuals, one completely in English and one completely in French. I was surprised to find the languages separated, since most U.S. manuals just roll the different languages up together into one booklet. (I wonder if these super-secret French Canadian manuals are where you find the ninja button combinations that let you set the language in games like, say, Time Hollow or Castlevania: Order of Ecclesia to French. It certainly doesn't appear to be in the U.S. manual anywhere.)
I'm wondering if Mexico gets the same cartridges we do. I rather suspect so. If so, how are they packaged? And what's it like buying games that play in English by default, that you have to fiddle with to turn to Spanish? Are there often games sold that only play in English (like the Ace Attorney games)? Are there any games produced that play only in Spanish (there are some monolingual French games)?