Jamieson, Fausset and Brown on 1 Corinthians 14:13

13. Explain, "Let him who speaketh with a tongue [unknown] in his prayer (or, when praying) strive that he may interpret" [Alford]. This explanation of "pray" is needed by its logical connection with "prayer in an unknown tongue" (1Co 14:14). Though his words be unintelligible to his hearers, let him in them pray that he may obtain the gift of interpreting, which will make them "edifying" to "the church" (1Co 14:12).