Paul words:
"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." 1 Cor 14:2
Alright, so later in the chapter we see this:
(1Co 14:9) So likewise ye, except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air.
(1Co 14:10) There are, it may be, so many kinds of voices in the world, and none of them is without signification.
(1Co 14:11) Therefore if I know not the meaning of the voice, I shall be unto him that speaketh a barbarian, and he that speaketh shall be a barbarian unto me.
So.. for that time.. speaking in an unknown tongue.. is not beneficial for the local church.. and it should be interpreted so that the rest of congregation know what is being said for the edification of the fellow brethren in that church.
The purpose of tongues in Acts 2.. has not changed for 1 Corinthians 12-14. It is still the same purpose for spreading the truth in other languages.
Point about the church at Corinth here.. they were getting the gift of speaking in tongues wrong. Not only this, they were getting the Lord's Supper wrong also. They were also upholding the likes of Paul and Apollos and following them.. rather than Jesus who was who Paul and co were pointing to. Corinth was also a port city.. and had all kinds in it.. with a church that was infected with the local climate of paganism and excess.
1Co 14:16 Else when thou shalt bless with the spirit, how shall he that occupieth the room of the unlearned say Amen at thy giving of thanks, seeing he understandeth not what thou sayest?
1Co 14:17 For thou verily givest thanks well, but the other is not edified.
1Co 14:18 I thank my God, I speak with tongues more than ye all:
1Co 14:19 Yet in the church I had rather speak five words with my understanding, that by my voice I might teach others also, than ten thousand words in an unknown tongue.
Gotta get the whole context here.