Here's an interesting question.
We know, as a matter of fact, Mass was conducted in Latin last time.
The problem is, a large majority of the Christians (ahem .. this is history qn .. no denominational squabbling pls) were uneducated. They probably don't understand any language beyond their native and local tongue (eg. an Italian in USA can speak Italian and English). So it is questionable whethere they understood what the priest was muttering every Mass. Or comprehend the hymns they sung.
So who taught them Latin?