This sign was a reversal of God’s judgment at the Tower of Babel. God’s judgment scattered and separated those people then, now He was uniting His people in the Spirit. At Babel no one could understand anyone else, but at Pentecost everyone could understand and hear God’s praises.
So, why did Peter choose Joel’s prophecy to explain what was happening? Yes, yes, he was saying what the Spirit was giving him to say. But, why Joel, in particular? Because of who were in that upper room, praying and waiting. Because of who were pouring out into the streets of Jerusalem, publicly prophesying and evangelizing in foreign languages. Women
Now, they were all (except for Thomas—where was he?) now gathered, fearfully, in a locked room in Jerusalem, not sure what to do, when something truly hair-raising happened.