Were the tongues spoken in the Bible known languages?
Yes. The tongues manifested in Scripture were known languages, not unintelligible speech. The Holy Spirit enabled believers to speak languages other than their own, and those languages could be understood by anyone present from that same linguistic background. This gift appeared only three times in Acts.
In Acts 19, Paul came to Ephesus and found certain disciples who had only received John's baptism and had not heard of the Holy Spirit. After they were baptized in the name of the Lord Jesus, Paul laid his hands on them, the Holy Spirit came upon them, and they spoke with languages other than their own and prophesied. Les teaches these are the only three times in the whole Book of Acts, in fact in all Scripture, where the Holy Spirit manifested Himself by giving out the gift of speaking a multitude of languages. But, as Les explains, they were known languages—languages that could be understood if somebody happened to be in the room from that same background.