What does 'tongues' in the plural mean in the Bible?
In the plural, tongues refers to known languages—the ability to speak more than one language. It was a gift for communicating the Gospel to people of a different language, and not every believer had it, since not all speak with tongues or interpret.
Les explains that in the list of gifts, “diversities” means the ability to speak more than one language, and that is what the word “tongues” in the plural always implies. It was known languages that enabled believers to communicate the Gospel to people who spoke a different language. In the same section, Paul asks whether all are apostles, all prophets, all teachers, all workers of miracles, all have gifts of healing, all speak with tongues, or all interpret—expecting the answer no. Les also notes that believers should desire earnestly the best gifts, and that even gifted service must be exercised with the love of God; otherwise they were better off staying home.
And God hath set some in the church, first apostles, secondarily prophets, thirdly teachers, after that miracles, then gifts of healings, helps, governments, diversities of tongues.