The Apostles most likely spoke Aramaic when speaking to people of the immediate area of Israel. When traveling beyond their immediate homeland then Greek was a language that was well known throughout the region and was most likely the common known language amongst people from different regions with different native languages. Paul spoke Hebrew (Acts 22:1-2) and wrote and debated in Greek and, being a Roman citizen, might have known some Latin as well.
That’s not to say that the Apostles were all equally fluent in Greek but given that it was the most widely used language at the time it would appear to be the most probable language that the Apostles used in their missionary work outside their own backyard.
