1 Corinthians 14:9-11

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.