Is unknown tongues a heavenly language or does it mean a language not known to people but for God?
The phrase "unknown tongue" only appears in the King James Version in six places in I Corinthians 14. If you look carefully you will notice that in all six cases "unknown" is in italics. This is because the word "unknown" doesn't appear in the Greek. The Greek simply says "tongue." The translators of the King James Version inserted the word "unknown" in an attempt to clarify the usage of the word "tongue" in this chapter. Unfortunately, some people have taken this translation and applied meaning to the phrase that is not usupported by the text.
First, let us allow the Bible to define what is meant by "tongue." The first recorded instance of speaking in tongues is found in Acts 2 when the Holy Spirit descended upon the disciples. "And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance" (Acts 2:4). We are not left guessing as to what is meant because immediately thereafter it is defined. "And there were dwelling in Jerusalem Jews, devout men, from every nation under heaven. And when this sound occurred, the multitude came together, and were confused, because everyone heard them speak in his own language. Then they were all amazed and marveled, saying to one another, "Look, are not all these who speak Galileans? And how is it that we hear, each in our own language in which we were born?" (Acts 2:5-8). When the disciples spoke in tongues, they spoke in the native language of the people present. Hence, speaking in tongues is nothing more that the ability to speak in other languages. What made this a miracle was the fact that the men speaking were not trained speakers in these languages. They were just Galileans from the poor, uneducated section of the country. Since Jesus' comission was for the disciples to "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit teaching them to observe all things that I have commanded you" (Matthew 28:19-20), you can see that the ability to speak in the native language of other nations was essential for the spreading of the gospel to all nations.
In I Corinthians 14:2 Paul says, "For he who speaks in a tongue does not speak to men but to God, for no one understands him; however, in the spirit he speaks mysteries." Here Paul is addressing those who who speak in a language (tongue) that is not known to those present. Such a person cannot be addressing the gathered audience, because no one understands him; only God knows what he is saying. He contrasts the gift of speaking in another language with the gift of prophecy. "But he who prophesies speaks edification and exhortation and comfort to men. He who speaks in a tongue edifies himself, but he who prophesies edifies the church" (I Corinthians 14:3-4). Prophesy has a purpose in the church, it allows men to learn of God's will for them. But speaking in a language that only the speaker knows only brings edification to the speaker because he is the only one who knows what he is talking about. As such, tongue speaking is showy, but useless in the assembly of the church. Prophecy is considered to be the better gift (I Corinthians 14:5) unless someone is able to interpret the tongue speaker's language so that others can benefit from the message.
This is not to say that speaking in other languages doesn't have its place. "In the law it is written: "With men of other tongues and other lips I will speak to this people; And yet, for all that, they will not hear Me," says the Lord. Therefore tongues are for a sign, not to those who believe but to unbelievers; but prophesying is not for unbelievers but for those who believe. Therefore if the whole church comes together in one place, and all speak with tongues, and there come in those who are uninformed or unbelievers, will they not say that you are out of your mind? But if all prophesy, and an unbeliever or an uninformed person comes in, he is convinced by all, he is convicted by all" (I Corinthians 14:21-24). Tongue speaking can help convict the unbeliever if he hears the message in his native tongue. But even with the unbeliever, tongue speaking is useless if the unbeliever doesn't understand what is being said. If he is addressed in a language that he does not know, it will sound like gibberish and he will leave convinced that the church is out of its mind.
Throughout the Scriptures, the emphasis is placed on spreading the gospel message, not in showing off gifts given from God. "For since, in the wisdom of God, the world through wisdom did not know God, it pleased God through the foolishness of the message preached to save those who believe" (I Corinthians 1:21). "For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes" (Romans 1:16). Modern-day tongue speaking redirects the focus from the message to the tool. Like the Corinthians of the first century, modern-day tongue speakers have lost track of the purpose the gifts were given. "But now, brethren, if I come to you speaking with tongues, what shall I profit you unless I speak to you either by revelation, by knowledge, by prophesying, or by teaching? Even things without life, whether flute or harp, when they make a sound, unless they make a distinction in the sounds, how will it be known what is piped or played? For if the trumpet makes an uncertain sound, who will prepare himself for battle? So likewise you, unless you utter by the tongue words easy to understand, how will it be known what is spoken? For you will be speaking into the air. There are, it may be, so many kinds of languages in the world, and none of them is without significance. Therefore, if I do not know the meaning of the language, I shall be a foreigner to him who speaks, and he who speaks will be a foreigner to me. Even so you, since you are zealous for spiritual gifts, let it be for the edification of the church that you seek to excel" (I Corinthians 14:6-12). Modern-day tongue speaking is more interested in the sound than presenting the melody. Hence, it fails to represent the true gift of tongues described in the Scriptures. "I thank my God I speak with tongues more than you all; yet in the church I would rather speak five words with my understanding, that I may teach others also, than ten thousand words in a tongue" (I Corinthians 14:18-19).
Therefore, an unknown tongue speaking is neither a heavenly language or a language exclusively used for speaking to God. An unknown tongue is a language that the listener does not understand. It is to be avoided because it serves no purpose.