For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him; howbeit in the spirit he speaketh mysteries.
Speaketh ... unto God (Romans 8:27) - who alone understands all languages.
No man understandeth. Generally, the few who can interpret tongues are the exception.
In the spirit - as opposed to "the understanding" (1 Corinthians 14:14).
Mysteries - unintelligible to the hearers, exciting wonder rather than instructing. Corinth, a mart for merchants from Asia, Africa, and Europe, would give scope for the exercise of the gift of tongues; but its legitimate use was in an audience understanding the tongue of the speaker, not in mere display.