After the Mexican War of Independence (1820) only 40% of Mexicans were able to speak Spanish

How did the Mexicans lose their language?

The population of the Indigenous people in Mexico were still larger than the colonist and Mestizos during 1821. The Indigenous people either spoke their own Native language as their first language while several only spoke it as their only language.

Some Mexicans began speaking Spanish during 1500's but a statement quoted below states that in general Mexicans began speaking Spanish in early 1700's only.  During mid-1500's Spanish population in Mexico was quite small and it probably took several decades or a century to have a large enough population.

Spain only began speaking Spanish Castilian as their main language in the late 15th century. During the invasion of Mexico in early 1500's, Spaniards were speaking their own regional language along with Spanish Castilian. 

 

Ostler doesn't provide any specific estimates on the percentage of Mexican citizens who spoke Spanish around the time of independence in 1821. He does note that 55 percent of Mexico's 6.7 million inhabitants in 1810 were "pure-blood Indians" who he implies — but does not say explicitly — largely spoke indigenous languages, 

 

 

Mexico started speaking Spanish since the Spanish colonization ordered it in 1st  Jan, 1714. there are still a few natives that speak Náhuatl, Maya, Otomí, Mixteco and Purépecha, which are native indigenous tongues, these located from central to south Mexico.