'Gift of Tongues' should be used only if there is an interpreter.
Pastorally Speaking, St. Paul in his first letter to the Corinthians rules out any speaking in public, during the assemblies, when there is no one to translate. The advice of St Paul that there should be an interpreter is always valid. Unfortunately, this is not followed by many prayer groups. The Aposle prefers to say five intelligible words than ten thousand in unknown and unintelligible tongues. (1 Cor 14:4-19). Nevertheless,some thinkers claim that the speaking in languages, even the with incomplete words or/and unintelligible sounds, is praise to the Lord (1 Cor14:4 ), since every tongue should praise the Name of Jesus and confess that he is in the Glory of the Father(see Philippians 2:11)Needless to say, that such a goft is restrictedto the sole beneficiary who praises the Lord without understanding himself (herslf)at times what he(she)says. Even the unintelligible-and not understood sounds- find somehow a "justification", for some thinkers, in the sense that they fulfill the words of St.Peter about our incapacity to pray. "So, the Spirit comes to assist us, praying himself in us, through unintelligible sighs"(see Romans 8:26).
As for Acts 2, I prefer to interpret that event as "common faith", rather than a language understood by every one in his own language. I prefer to say that all who heard Peter, embraced the same faith. The Holy Spirit gave every listener the gift of faith in Jesus Christ.