On "UnknownTongues"

I am working on a paper for a leadership class I am taking at church. The topic is "unknown tongues" mentioned in 1 Corinthians 14. I am looking at a rule in hermeneutics which our text book calls "The Law of First Mention", the law is the first mention of any given subject gives the key to its subsequent meaning. So using this "law" I am to refute the idea that charismatic theology imposes on "unknown tongue" meaning a language given for a heavenly language. When I go to Acts 2:4-11 I see the first mention of tongues at the inception of the church. The apostles were given the gift of tongues in order to share the Gospel at that unique time. Several nations and people were represented there because of their journey to Jerusalem for Pentecost, the men spoken in recognizable dialects of each person represented there, thus the Gospel spread back to their homelands. When I go to I Corinthians 14 I see the term "unknown tongues", thus Charismatic theology implies that this tongue is given for edification of the one speaking it and it is directed to God. I would disagree based on the context of the passage. Prophecy is also mentioned and its purpose is for edification of the saints where as an unknown tongues is for personal edification or it only edifies the speaker personally because there is no interpreter nor anyone that understands that tongue. The words used in the Acts and Corinthians passage for tongue are defined as dialects and languages, not unknown languages or heavenly languages. In verse 2 of Corinthians 14 we see that it is a mystery and no man understands. I would conclude that it is not a mystery because it is a heavenly language unknown to man and meant for God, but it is a mystery because there is no interpreter or anyone that speaks the dialect. It edifies the man because he is the only one that understands what is being said.