There seems to be nothing more astonishing or fascinating than the idea of speaking in tongues ― a language we have never learned, or the language of “angels”, as St. Paul mentions it in (1 Cor 13:1).
The charism of tongues is primarily a gift for prayer ― the Holy Spirit speaks directly to God through us ― even if we don’t understand our speech. “One who speaks in a tongue speaks not to men but to God” (1 Cor 14:2). The charism of tongues is a ‘gateway’ to other gifts and life in the Spirit.
St. Thomas Aquinas considered the charism of tongues paramount to evangelization – the sharing of the faith. For example, one friend at university was enabled by the Holy Spirit to speak to a group of French students, even though he knew no French. He was not only able to help them but also to share his faith with them. The charism of tongues may simply signal the presence of the Holy Spirit, though one may receive the Holy Spirit without speaking in tongues.
The most obvious example of speaking in tongues happened at the very first Pentecost (Acts 2:4-11), when the disciples shared the gospel and all the people present heard “in their own tongues the mighty works of God!” In public worship, a person might prophesy in tongues, and another person might interpret the speech for the group to hear God’s word. The interpretation of tongues is another charism.