Answer: Did all soldiers in the Roman Army actually speak Latin, or were only the commanders fluent?”
It somewhat depends on the period but, in general, yes.
This is a bit more interesting topic than you might think. One thing to understand is that the Empire was a military dictatorship. The army was a club, a gang in some sense, that ruled over the civilian population (not entirely unlike a modern gang ruling over a neighborhood and extorting money from the local businesses). Originally the army was composed exclusively of men from the city of Rome itself ruling over the conquered peoples of the lands around the Empire. Citizenship was gradually extended over the Empire but the army remained a very exclusive club. A defining characteristic of the army even in later times was that it was exclusively Latin-speaking. Though not everybody was natively Latin-speaking, to become a member of the club, you had to become a Latin speaker.
This becomes very interesting in late classical times. In the Western Roman Empire, the army came to be manned entirely by mercenary armies, mostly Germanic tribes referred to as foederati. These tribes were not natively Latin-speaking, of course, but they nevertheless used Latin (broken Latin to be sure) in their operations. In the Eastern Roman Empire, Latin was a small minority language spoken only in a few regions to the north. Nevertheless, the army still drew heavily from this group and the army remained a Latin club. This situation continued until the 6th century but then things finally reached a point where the Greek majority in the northeastern Mediterranean came to dominate and the Latin minority was pushed aside.
Step-by-step explanation: