1 Corinthians 14:10

There are, it may be, so many kinds of voices in the world, and no kind is without signification. If then I know not the meaning of the voice, I shall be to him that speaketh a barbarian, and he that speaketh will be a barbarian unto me.

These verses are a recapitulation of the argument in 1 Corinthians 14:7-9, the conclusion being that any kind of jargon or gobbledegook, such as tongues, which cannot be understood by the hearers, is condemned.

Barbarian ... in ancient times meant merely one who did not speak Greek. Paul encountered some of these "barbarians" on his mission tours, namely, at Malta and at Lycaonia (Acts 14:11); and significantly Paul did not understand the dialect of the Lycaonians, this being another reason to suppose that Paul's gift of tongues did not include the gift of speaking in languages he had never learned, but was rather for private encouragement.