The amazing thing about Pentecost was not that the Apostles were speaking in tongues, but that they were UNDERSTOOD by all the folks who had come up to Jerusalem from all over the eastern Med - "Parthians, Medes and Elamites; residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the parts of Libya near Cyrene, Romans, Cretans and Arabs . . . "
THAT's why Koine Greek was necessary. It filled much the same function as English today, with some speaking it better than others. And it's interesting that the LXX - the Greek Old Testament - is the one most often quoted by Our Lord.
In His providence, God had prepared the Eastern Mediterranean beginning more than 500 years before the birth of Christ.
First, when the northern and southern kingdoms of Israel were destroyed Jews were dispersed throughout the region.
Second, just over 300 years before Christ, Alexander the Great established Greek speaking enclaves throughout the region. Greek became the standard language of commerce which the Romans left in place.
After Alexander the Romans replaced the Greeks and established Pax Romana, the Roman Peace. They built, maintained, and guarded roads throughout their empire. These are the roads employed by Peter, Paul and the others. In most of the places Paul visited he would start at the Synagogues. The first church services in Rome were held in synagogues which had been wrested from the Jewish leadership by Jewish converts. Home churches in Rome did not occur until after the ejection from and return to Rome by the Jews. The returning remnant found the synagogues destroyed.
200-250 years B.C. Jews who were in Alexandria, Egypt translated the LXX, Septuagint from Hebrew. This was an attempt to accommodate the dispersed Jews who were unable to read Hebrew.
By the time Jesus was born the table was set for the spread of Christianity throughout the Western world. Paul and the Apostles were blessed with peace, roads, Jewish communities with copies of LXX in hand, and the common Greek language.