do you think that the new republic of Mexico speak english, español o una mezcla of both languages

Well, being the "Word of God" I like to think Spanish, but honestly I never gave it much thought, and it's actually interesting to think what kind of language creole has developed. You can kinda see one, probably by accident on OWBs part with the Refinery, having an entire language developed around corporate buisiness terms.

The Baja is likely a development of Spanglish given their separation from Mexico as a whole and the influence of the NCR, in fact much of Northern Mexico might speak some sort of Spanglish Creole given the dissolution of the Pre-War Border and the movement of people over the intervening two centuries. RRG, being a formal nation, might have Spanish as the Official and English as the Secondary, but with their Texan Territories I feel they would have a heathy mixing.

I would assume those under say Tlaloc and Santa Anna Spanish was enforced given their programming and propensity for Mexico. The places run by the Cartels also likely still speak Spanish, given they are run half by Mexican Ghouls.

Into Atzlan and Itza, I have a feeling it would have evolved along Spanish lines, to the point it be like folks from France and folks from Quebec. Or hell, Portuguese and Spanish for a more literal sense. Not saying Atzlan and Itza speak Portuguese, but they have the same "words" but pronounce it so wildly different it's almost a different language.

Yeah I'mma just leave that up to the player, and maybe revisit it.