FUN FACT: Romans actually put very little emphasis on teaching the dirty provincials Latin. They simply gave and posted all their laws and proclamations in Latin, and if the provincials didn't understand it, too fucking bad, ignorance is no excuse for not breaking the law.

This, obviously, put those who DID know Latin at a distinct advantage, not only for avoiding trouble themselves, but also for getting their fellows out of trouble by knowing what the law ACTUALLY was. It created a kind of lower provincial elite (or middle-class, if one prefers) with a self-interest in learning Latin (and, ideally, literacy). This relatively soft hand, as opposed to 19th century nations beating children for speaking 'dirty' languages, actually was very effective at establishing Latin in conquered territories. Language was one of the more deeply rooted aspects of the Empire in the West for this reason.