Since Acts 2:4-11 teaches that when men spoke in tongues (i.e., "as the Spirit gave the utterance"), they spoke actual, understandable human languages, is it possible that any other cases of tongue speaking were in languages which could not be understood (e.g., 1Co 14:2 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.
1Co 14:3 But he that prophesieth speaketh unto men to edification, and exhortation, and comfort.
1Co 14:4 He that speaketh in an unknown tongue edifieth himself; but he that prophesieth edifieth the church. AND WHAT IS THE DIFFERENCE? reply.
One is the 'tongues of humans' and another 'tongues of angels' (1Corinthians 13;1,beginning). These are only a 'sign' for 'unbelievers'. But they are edifying to oneself uttered in silence, having not the gift of 'interpretation' and they are edifying to the church only with 'interpreting' them, for everyone to understand it's meaning and at the end, the church say 'Amen' together and these are a 'sign' for a 'believer' (1 Corinthians 14;22 and 16).
While 'Prophesying' on the other hand 'built up' the church, unless one bring the church, some 'revelation' or 'knowledge' or 'prophecy' or 'teaching' (verse 4,end and 6,end).
GOD bless everyone in the name of LORD JESUS CHRIST.