At Pentecost when the Holy Spirit came upon the Apostles they spoke in Tongues and everyone understood them in their own language. It seems later the gift of Tongues changed to something secret and required an interpreter. What happened?
The purpose of the gift of tongues is to communicate a message from God. So everyone can know the message is truly from Him, He enables a person to speak the message in a language he or she doesn’t understand, and then enables another person to translate it into the common language of the listeners. In this way two different miracles confirm the validity of the message. In 1 Cor 14:27-28 Paul explained this.
If anyone speaks in a tongue, two—or at the most three—should speak, one at a time, and someone must interpret. If there is no interpreter, the speaker should keep quiet in the church and speak to himself and God.
This tells us under normal circumstances the gift of tongues has always required an interpreter. That’s why there’s also a gift of interpretation (1 Cor. 12:10) But at Pentecost the languages the disciples spoke in were the native languages of their listeners (Acts 2:3-11) so no interpreter was necessary.