Biblical Tongues Wasn't Always Only for Preaching

The Biblical gift of tongues did not always mean that the speaker spoke a language that was understood by his audience, as was the case in Acts 2. Otherwise there would not have been a need for the gift of the interpretation of tongues. 

The gift of the interpretation of tongues was not a natural ability to translate a learned language, but a supernatural ability to interpret an unknown language - otherwise Paul wouldn't have said that a speaker of an unknown tongue may pray that he may interpret.   

Even when no interpreter was present, the tongues which were being spoken in the church at Corinth were nonetheless legitimately given through the Spirit, or else if the tongues were invalid, Paul would not have urged them to pray to interpret what had been spoken, nor would he have said that the speaker of the unknown tongue could continue speaking in the unknown tongue to himself and to God, nor would he have said that the speaker of the unknown tongue edified himself. 

So in Bible-times it often happened that tongues were spoken which were unknown to both the speaker and his audience - and in such cases the tongue itself was not considered to be mere babble nor demonic in origin. "He verily giveth thanks well," said Paul of the speaker of an unknown tongue. 

Paul's only issue with such tongues, if used to address a congregation, was that it was not understood. There was no question that the tongue itself may have been mere babble or that it may have been demonic just because no-one present understood it. There were still a number of valid uses for tongues even when they were unknown: someone else with the gift of interpretation should interpret; the speaker himself could pray that he may interpret; or he can refrain from addressing the congregation in the tongue and instead continue to speak it unobtrusively to himself and to God.