Joel 2:28&29
28*And it shall come to pass afterward, that I will pour out my spirit upon all flesh; and your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions:
29*And also upon the servants and upon the handmaids in those days will I pour out my spirit.
Mark 16:15-18
15*And he said unto them, Go ye into all the world, and preach the gospel to every creature. 16*He that believeth and is baptized shall be saved; but he that believeth not shall be damned. 17*And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; 18*they shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.
Acts 2:1-13
1*And when the day of Pentecost was fully come, they were all with one accord in one place. 2*And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. 3 And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. 4 And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
5*And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven. 6*Now when this was noised abroad, the multitude came together, and were confounded, because that every man heard them speak in his own language. 7*And they were all amazed and marvelled, saying one to another, Behold, are not all these which speak Galilaeans? 8*And how hear we every man in our own tongue, wherein we were born? 9*Parthians, and Medes, and Elamites, and the dwellers in Mesopotamia, and in Judaea, and Cappadocia, in Pontus, and Asia, 10*Phrygia, and Pamphylia, in Egypt, and in the parts of Libya about Cyrene, and strangers of Rome, Jews and proselytes, 11*Cretes and Arabians, we do hear them speak in our tongues the wonderful works of God. 12*And they were all amazed, and were in doubt, saying one to another, What meaneth this?
13*Others mocking said, These men are full of new wine.
Acts 10:24-26
44 While Peter yet spake these words, the Holy Ghost fell on all them which heard the word. 45 And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost. 46 For they heard them speak with tongues, and magnify God.
1Corinthians 14:22
22 Wherefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying serveth not for them that believe not, but for them which believe.
Peter said to the crowd that which was witnessed was the prophecy of the prophet Joel and I do not see anywhere in the scriptures where it says that God will understanding of unknown tongues to unbelievers. But we can see that over and over the scriptures do say that God will cause them to speak in new tongues ( tongues unknown to those speaking. Jesus didn't say that the believers would understand different languages but He did say that they would speak in new tongues. The miracle of Acts 2 is explained perfectly by Paul in 1Cor 14:22.
As far as the Apostles speaking Galilean, I was not aware that there was such a language. The common languages in Jesus' day in Israel were Aramaic (Aramaen) and Greek, and some Hebrew. I find it much more sound to place the speaking and not the hearing into a supernatural phenomenon.