1 Corinthians 14:9-11

So also you, unless you utter by the tongue speech that is clear, how will it be known what is spoken? For you will be speaking into the air. There are, perhaps, a great many kinds of languages in the world, and no kind is without meaning. If then I do not know the meaning of the language, I will be to the one who speaks a barbarian, and the one who speaks will be a barbarian to me.