I would guess that whenever they were "colonized" by the Spaniards. Afterall, the Spanish language starts with the Spaniards. And it's not like the Mexicans starting speaking spanish out of the blue one day.
== == The largest spanish speaking country Mexico -- mexico
No. It is a Spanish-speaking city in Mexico.
Mexico is the Spanish-speaking country closest to the United States.
Mexico is the Spanish speaking country that borders the US to the south.
Mexico is the largest Spanish speaking country.
The most populous Spanish-speaking country in the world is Mexico.
the Spanish speaking country only in north America is i think Mexico.
Yes, the main language in Mexico is Spanish.
Mexico has the largest Spanish speaking population
A Spanish-speaking country that starts with "M" is Mexico.
Mexico borders the US to the south.
Assuming this is a clue for a US-based crossword puzzle, then the answer is Mexico.