The spiritual gift of speaking in other tongues (languages) was given to the church (I Cor. 12:1-11; I Cor. 12:28-30) for the purpose of providing a sign to "them that believe not" (I Cor. 14:22).
In speaking of tongues in the Scriptures, the word "unknown" is always in italics, indicating that the word has been added to clarify that the tongue (language) spoken was unknown to the speaker but a tongue (language) that could be understood by those who spoke that tongue (language) -- and one that could be interpreted by one knowing the spoken language and the language of the hearers and, also, interpreted by those having the gift of interpretation (I Cor. 14:27-28; I Cor. 12:10).
The gift of tongues was not given to each believer in the church.
I Cor. 12:8-11:
For to one is given by the Spirit the word of wisdom; to another the word of knowledge by the same Spirit;
To another faith by the same Spirit; to another the gifts of healing by the same Spirit;
To another the working of miracles; to another prophecy; to another discerning of spirits; to another divers kinds of tongues; to another the interpretation of tongues;
But all these worketh that one and the selfsame Spirit, dividing to every man severally as he will.
When the church came together before the gift of tongues was "done away" (I Cor. 13:10), not more than three men were permitted to speak in other tongues, one at a time -- and it was required that someone interpret the speaking. If there were no interpreter, "let him keep silence in the church" (I Cor. 14:23-28).
Women were commanded to "keep silence in the churches" (I Cor. 14:34-35); therefore, women were not permitted to speak in tongues in the church.
During the time that the gift of tongues was in existence, those desiring spiritual gifts were exhorted to excel in gifts that edify (build up) the church (I Cor. 14:12). The more desirable gift was the gift of prophecy which was for the benefit of believers and edifies (builds up) the church (I Cor. 14:1, 3, 4, 22).
Speaking in tongues, unless interpreted, did not edify (build up) the church (I Cor. 14:1-5); instead, the tongues speaker edified (built up) himself (I Cor. 14:4).
In the words of Paul, "If any man think himself to be a prophet, or spiritual, let him acknowledge that the things that I write unto you are the commandments of the Lord" (I Cor. 14:37).
