The Romans when expanding (The biggest territorial extent around 117AD) were quite happy to set up a government in a conquered town for example and still allow the population to practice their religion and language as well as festivals and other traditions. Although at the time of the empire it was expected if you are under Roman rule that you respect and admire the Emperor and any bad mouthing could find you executed! (Especially under the rule of the Julio-Claudians, including Augustus) Latin was not imposed officially under Roman rule yet traders and merchants might have found it handy when dealing with such people speaking Latin. In fact, if the Romans conquered a city and they liked their pagan Gods that this society worshipped than the Romans would throw these God's into their own Pantheon of God's to worship! In essence the Romans were not too fussed about their conquered people learning Latin and worshipping their God's yet wanted some respect to various figures in the Roman world.