Jamieson, Fausset and Brown on 1 Corinthians 14:2

2. speaketh ... unto God--who alone understands all languages.

no man understandeth--generally speaking; the few who have the gift of interpreting tongues are the exception.

in the spirit--as opposed to "the understanding" (1Co 14:14).

mysteries--unintelligible to the hearers, exciting their wonder, rather than instructing them. Corinth, being a mart resorted to by merchants from Asia, Africa, and Europe, would give scope amidst its mixed population for the exercise of the gift of tongues; but its legitimate use was in an audience understanding the tongue of the speaker, not, as the Corinthians abused it, in mere display.