1 Corinthians 14:9-11

14:9 It is the same for you. If you do not speak clearly with your tongue, how will anyone know what is being said? For you will be speaking into the air. 14:10 There are probably many kinds of languages in the world, and none is without meaning. 14:11 If then I do not know the meaning of a language, I will be a foreigner to the speaker and the speaker a foreigner to me.

1 Corinthians 14:16

14:16 Otherwise, if you are praising God with your spirit, how can someone without the gift ¹  say “Amen” to your thanksgiving, since he does not know what you are saying?

1 Corinthians 14:21-22

14:21 It is written in the law: “By people with strange tongues and by the lips of strangers I will speak to this people, yet not even in this way will they listen to me,” ²  says the Lord. 14:22 So then, tongues are a sign not for believers but for unbelievers. Prophecy, however, is not for unbelievers but for believers.

[14:16]  ¹ tn Grk “how can someone who fills the place of the unlearned say ‘Amen.’”

[14:21]  ² sn A quotation from Isa 28:11-12.