Matthew is thought to have written his version of the Gospel in Greek. As a tax farmer for the Romans, he would have roamed far and wide, and anyone who was anyone in those days, knew Greek (Koine), especially in Galilee. Greek was the language of business and culture during those times. The only version of the scripture, and not a very early one at that, from which we got the KJV of Matthew, was written in Greek, not Aramaic or Hebrew.
IMHO, the church was built upon the foundation of Christ, not Peter.
Matthew is thought to have written his version of the Gospel in Greek. Papias says very clearly that Matthew wrote in "Hebrew".
However, I'm not sure it really matters. Matthew is relating a conversation. The conversation took place in Aramaic. We can prove that from the Bible. Case closed.