So, the language of Mexico is Spanish? Not quite

[Xipe Totec]

3. More to the point of your reply...

In 1970, or thereabouts, when my father was applying for me to acquire US citizenship through him, we were at the El Paso border crossing waiting to cross and in front of us was a couple, blond, blue eyed, late 50's early 60's. He was dressed in coveralls, a plaid flannel shirt, and straw hat. She was dressed in a gingham dress and bonnet. When they reached the counter, the immigration officer asked: "How may I help you?" in English, naturally. They looked at each other confused and he says in Spanish: ¿Que dijo?

Immediately the officer switched to Spanish and everything went forward normally.

It was then I learned, from my father, that there were colonies of Mennonites living in Chihuahua that only spoke some dialect of German, and Spanish as a second language.

So, yea, Plautdietsch most likely.