1 Corinthians 14:7-13

Yet even lifeless things, whether flute or harp, when producing a sound, if they do not produce distinct [musical] tones, how will anyone [listening] know what is piped or played? And if the [war] bugle produces an indistinct sound, who will prepare himself for battle? So it is with you, if you speak words [in an unknown tongue] that are not intelligible and clear, how will anyone understand what you are saying? You will be talking into the air [wasting your breath]! There are, I suppose, a great many kinds of languages in the world [unknown to us], and none is lacking in meaning. But if I do not know the meaning of the language, I will [appear to] be a foreigner to the one who is speaking [since he knows exactly what he is saying], and the one who is speaking will [appear to] be a foreigner to me. So it is with you, since you are so very eager to have spiritual gifts and manifestations of the Spirit, strive to excel in ways that will build up the church [spiritually]. Therefore let one who speaks in a tongue pray that he may [be gifted to] translate or explain [what he says].