1 Corinthians 14:4-6
1Co 14:4 The one speaking in another language builds himself us, but the one prophesying builds up the church.
1Co 14:5 I wish all of you spoke in other languages, but I wish even more that you prophesied. The one prophesying is greater than the one speaking in other languages, unless he interprets so that the church may be built up.
1Co 14:6 But now, brothers, if I come to you speaking in other languages, how will I make you gain from that, unless I speak to you with a revelation or knowledge or prophecy or teaching?
tongues are for understanding
The linguists within the Corinthian fellowships had a viable ministry, so Paul wanted them to continue. But the ministry in other languages was not directly edifying to those members of their congregations who could not speak those languages. So, Paul encouraged prophesying instead. The exception would be in there was an interpretor present. Then, the message — whether it be an unveiling of a mystery, or exposition of scripture (knowledge), or a prophetic word of warning or encouragement, or a systematic teaching — could be understood by all the listeners.
The popular idea that Paulo refers to some kind of exstatic utterance is clearly a misconception. Why would the Holy Spirit give someone a supernatural gift of speaking in a language no one could understand? It goes against the whole point that Paul is making. He encourages excercising gifts that your listeners can underrstand.
LORD, give us the wisdom to minister to others in such a way as to lead them to understand you.