Albert Barnes Commentary Acts 21:37

"And as Paul was about to be brought into the castle, he saith unto the chief captain, May I say something unto thee? And he said, Dost thou know Greek?" — Acts 21:37 (ASV)

May I speak unto thee? May I have the privilege of making my defense before you; or of stating the case truly—the cause of my accusation, of this tumult, etc.?

Canst thou speak Greek? This implied that if Paul could, he might be permitted to speak to the commander.

The Greek language was then almost universally spoken, and it is not improbable that it was the native tongue of the commander. It is evident that he was not a Roman by birth, for he says (Acts 22:28) that he had obtained the privilege of citizenship by paying a great sum.

The language the Jews spoke was Syro-Chaldaic. Since the commander took Paul to be an Egyptian Jew (Acts 21:38), he also supposed from that circumstance that Paul was unable to speak Greek.