Question: "What is the spiritual gift of interpreting tongues?"
Answer: Along with the gift of speaking in tongues, another spiritual gift mentioned in the list found in 1 Corinthians 12:10 is the gift of interpreting tongues. The gift of interpreting tongues is the ability to translate a foreign language into the language of the hearers. The gift of interpreting tongues is always alongside the gift of speaking in tongues. It is a separate gift, but it is always used in conjunction with the gift of speaking in tongues.
A person with the gift of interpreting tongues could understand what a tongues-speaker was saying even though he did not know the language that was being spoken. This is what distinguishes the spiritual gift from the natural gift of being able to understand and speak a variety of languages. The tongues interpreter would then communicate the message of the tongues speaker to everyone else, so all could understand and benefit from the truth being spoken. The tongues were known languages, not ecstatic utterances. According to the apostle Paul, and in agreement with the tongues described in Acts, speaking in tongues is valuable to the one hearing God’s message in his or her own language, but it is useless to everyone else unless it is interpreted/translated. Paul's concern is edification of the church (1 Corinthians 14:5, 12).
Paul’s conclusion regarding tongues that were not interpreted is powerful: “But in the church I would rather speak five intelligible words to instruct others than ten thousand words in a tongue” (1 Corinthians 14:19). There is no benefit to others in hearing something they cannot understand. More importantly, there is no benefit, and much harm, done in churches where the speaking and interpreting of a tongue brings forth that which does not line up with Scripture or which cannot be verified in Scripture.
Paul was also concerned about order in worship. His concern was that everything is done for edification of the church. He goes on to say that there should only be two or three speaking in a tongue and one should interpret. If there is no interpreter present, then one should be quiet (1 Corinthians 14:26-28). The temporal nature of the gift of tongues assumes that the gift of interpretation of tongues was also of a temporal nature. If the gift of speaking in tongues were active in the church today, it would be performed in agreement with Scripture. It would be a real and intelligible language (1 Corinthians 14:10). It would be for the purpose of communicating God's Word to a person of another language (Acts 2:6-12), and it would also be in accordance with 1 Corinthians 14:33, “For God is not the author of confusion, but of peace, as in all churches of the saints.”