I am under the impression that the Roman military contained soldiers from regions throughout the Empire, many of whom presumably didn't speak Latin as a first language. Were these people taught to speak at least basic Latin as part of their training, were they expected to pick it up on their own, or were they simply placed under the command of people who spoke their language?
Legions could only be recruited legally from citizens. So legionaries would know Latin perfectly fine. Auxiliaries are another story, but in the Principate they were commanded by Roman officers, who would've known Latin. Knowledge of Latin was quite widespread, and if you didn't know Latin there was a good chance you knew Greek