When the disciples spoke in ‘other tongues’ after being filled with the Holy Spirit, they were not speaking an unintelligible, heavenly language. They were speaking human dialects or languages of other nations. For “every man heard them speak in his own language” (Acts 2:6).
This “gift of tongues” was miraculous because the disciples had not previously known the languages in which they spoke. This helped to further the spread of the gospel and to reinforce that salvation was a gift to all people.