Were the tongues spoken in the Bible known languages or unknown?
The tongues spoken in Scripture were known languages, not unknown languages. They were languages that could be understood if someone happened to be in the room from that same background. In Acts, the Holy Spirit manifested Himself by giving the gift of speaking a multitude of languages.
Les teaches that in Acts 19, when Paul laid his hands on the twelve men at Ephesus, the Holy Spirit came upon them, and they spoke with languages other than their own and prophesied. He points out that this is one of only three times in the whole Book of Acts—indeed in all Scripture—where the Holy Spirit manifested Himself by giving out the gift of speaking a multitude of languages. Les emphasizes that these were known languages, languages that could be understood if somebody happened to be in the room from that same background.
And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues, and prophesied.