For many years I was taught that everyone in Israel, at the time of Jesus' ministry, all spoke Greek.

But then I read:

Acts 21:37 And as Paul was to be led into the castle, he said unto the chief captain, May I speak unto thee? Who said, Canst thou speak Greek?

Why would the chief captain be surprised that Paul could speak Greek, if they all spoke Greek?

Then, if they all spoke Greek, why then did Paul speak to the people in Hebrew?

Acts 21:39b  and, I beseech thee, suffer me to speak unto the people.

  40 And when he had given him licence, Paul stood on the stairs, and beckoned with the hand unto the people. And when there was made a great silence, he spake unto them in the Hebrew tongue, saying,

Acts 22:1 Men, brethren, and fathers, hear ye my defence which I make now unto you.

  2 (And when they heard that he spake in the Hebrew tongue to them, they kept the more silence: and he saith,)

Then the superscription on the cross:

Luke 23:38 And a superscription also was written over him in letters of Greek, and Latin, and Hebrew, THIS IS THE KING OF THE JEWS.

Perhaps not everyone spoke Greek, perhaps some only spoke Hebrew or Latin?