The state language of the Byzantine Empire was …

The state language of the Byzantine Empire was Greek. Greek was the native language of the vast majority of the inhabitants of the empire. In this language, laws were written and promulgated, correspondence was conducted, and the command of armies was carried out.

It is worth noting that until a certain time the state language was Latin, when the unity of the Western and Eastern empires was recognized.

Later, Byzantium became an independent state. The Greek language became the state language, but the study of Latin was not abandoned in Byzantium either.