Something miraculous happened on the Day of Pentecost - some people started speaking in ways that were foreign to them. This in turn enabled some other people to begin hearing what they had to say. Just because you speak the same language as someone else doesn't mean that they will actually hear what you are saying. Being able to actually hear what is being said, through your own biases, presuppositions and social and religious conditioning is a wonderful spiritual gift.
Have you ever heard someone say something and you understand their words, you hear what they are saying, but then you realize that you don't understand what they mean by their words? This is what happened at Pentecost. People heard the good news about Jesus in their native language, but asked, "What can this mean?" At the center of Pentecost lies a revelation of meaning.