Emperor's of Rome podcast addressed this in a Q&A episode.
From memory the answer is that top level generals would almost always speak Greek and Latin, mid level commanders would speak either Greek or Latin adequately as well as the local language of the troops they were commanding.
Found it
Emperors of Rome: Episode CVIII - A Lesson in Latin II Starting from: 00:08:02
Episode webpage: http://www.latrobe.edu.au/marketing/assets/podcasts/emperors/181128-latin02.mp3
Media file: http://www.latrobe.edu.au/marketing/assets/podcasts/emperors/181128-latin02.mp3#t=482