No matter how hard you try to spin it in your quest to restore the Imperial Roman Catholic Church, there is nothing particularly sacred about the Latin language.
First century Legions were raised in Italy and lived in mobile field encampments, not in towns. Their life was intwined in the military and in military comraderie. Why they would have bothered learning the language of peoples they barely considered human, and with whom they had little interaction is beyond me. Do you have some evidence of this from history? Did C. Quinctilius Varus' three legions speak German? I doubt it. They had taught the Germans they needed to interact with, like Arminius (Hermann the Cherusker) to speak Latin.
The natural reaction of visitors is to speak to the natives in your native language, not theirs, and hope they respond. Generally, when you are the occupation army, and they the occupied, they do.
Occupying Army's tend to learn native words shouting out to surrender and curse at them, and for getting food and sex. And that's it. "Spazierengehen fraeulein?"
How many Americans in Okinawa have comeback fluent in Japanese? How many in the Philipines speak fluent Tagalog?