We always hope that we will have guests at our worship services. With that in mind, Paul said,
“But if all prophesy, and an unbeliever or an uninformed person comes in, he is convinced by all, he is convicted by all. And thus the secrets of his heart are revealed; and so, falling down on his face, he will worship God and report that God is truly among you” (1 Corinthians 14:24-25).
Paul spoke of prophesying (preaching by divine inspiration), as opposed to speaking in a tongue, when a guest might not know what language the speaker was speaking, and so the guest would not derive any benefit.
However, if he went and heard preaching in his language, then he could know the truth of God and such a person may be converted, or at least he would confirm that you are God’s people.