[PGR88]

What language did Belisarius speak?

[Antoninus]

Given he was from Illyricum, he probably spoke both Latin and Greek like his countryman, Justinian. Illyricum was one of the best recruiting grounds left in the Empire and a core of Latin-speaking descendants of Roman colonists lived there. He may have also spoken a local Thracian dialect.

The language of the Roman Army at the time was also an amalgam of Latin and Greek (see the Strategicon of Maurice which has commands in Latin, but was written in Greek.)