Except that we are not talking about unbelievers.
The gift of supernaturally speaking in a foreign language (which had never been learned by the speaker) was evidence that God the Holy Spirit was empowering that person. This was a sign to unbelieving Jews that the Gospel was from God. And that is why the apostles and disciples were enabled to speak in approximately 15 languages and dialects on the Day of Pentecost.
In the law [the OT] it is written, With [men of ] other tongues and other lips will I speak unto this people; and yet for all that will they not hear me, saith the Lord. Wherefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying serveth not for them that believe not, but for them which believe. (1 Cor 14:21,22).
For with stammering lips and another tongue will he speak to this people [the Jews]. To whom he said, This is the rest wherewith ye may cause the weary to rest; and this is the refreshing: yet they would not hear. (Isa 28:11,12)
In spite of the miracle on the Day of Pentecost, Israel (as a whole) did not believe the Gospel. But this same gift of tongues was being manifested in the apostolic churches, and 1 Cor 12-14 was written TO CORRECT THE ABUSES of this gift, as well as teach us about the various gifts of the Spirit.