In Acts of the Apostles, those who speak in tongues are found to be speaking in foreign languages. It is clear in Acts 2:4 that it is the Holy Spirit who enables people to speak in tongues. If the Holy Spirit inspires an individual with this gift, the words spoken need not be understood by the person speaking. In his first letter to the Corinthians, St. Paul writes, "For one who speaks in a tongue does not speak to human beings but to God, for no one listens; he utters mysteries in spirit." That he used the phrased 'utters mysteries in spirit' has led scholars to understand this to mean unrecognizable utterances and not merely foreign languages.
In 1 Cor 14, St. Paul points to the importance of knowing what is said when speaking tongues. He prods the reader to pray for the gift of interpretation, for what good is the message without understanding? Certainly he is not suggesting that a Greek pray to understand Aramaic, or vice versa, but that the language spoken could not be understood by anyone without the supernatural gift of of interpretation. Note that St. Paul also does not appear to separate the occurrences of speaking in tongues as found at Pentecost and that of the Corinthians. A foreign tongue or a spiritual one are each considered 'speaking in tongues'.
In a homily on 1 Corinthians, St. John Chrysostom writes:
"This is the rule constantly followed by St. Paul -- to give preference to the gifts that make for the edifying of the Church. someone will ask, 'Is it possible for someone to speak in tongues without speaking to the benefit of his brethren?' Listen: those Christians do speak [in tongues], but what they say is less helpful to the edification, exhortation and consolation of souls than the gift of prophecy. Both [prophets and those who speak in tongues] are acting as the voice of the Holy Spirit who moves and inspires them; but what the prophet says is useful to those faithful who hear him, whereas the gift of tongues does not lead to understanding unless the hearers themselves have received the same supernatural gift."
In 1 Cor 14, St. Paul shares with the Corinthians that, "I thank God that I speak in tongues more than you all; nevertheless, in church I would rather speak five words with my mind, in order to instruct others, than ten thousand words in a tongue." He goes on to reiterate the importance of educating others in the Christian and to do only those things which edify the Church. In 1 Cor 39-40, he ends the chapter with:
"So, my brethren, earnestly desire to prophesy, and do not forbid speaking in tongues; but all things should be done decently and in order."
This was restated in Lumen Gentium at the Second Vatican Council:
"Extraordinary gifts are not to be rashly desired, nor is it from them that the fruits of apostolic labors are to be presumptuously expected."
Today when many people hear about speaking in tongues, they picture non-Catholic Christians babbling inaudibly in unison as part of their Sunday worship. St. Paul made it clear that not all will have the gift and the importance of interpretation for the edification of the Church. Clearly, not every person in a congregation would have the gift of speaking and/or interpreting tongues. When people are pressured to 'speak in tongues' or use their 'gift' to draw attention, it is obvious that the focus has turned from serving God and His Church to serving mammon. Because of this, some Catholics have a great deal of misgivings about speaking in tongues. For many, it seems more of a display of great theatrics by some hysterical, albeit well-intentioned Christians.
In summary, to speak in tongues can be now as it was 2000 years ago, either a true foreign language or utterances only God and those with the gift of interpretation can understand. All gifts are to be used for edification of the Church and to help others in their understanding of the Christian faith. There certainly are Christians today who are blessed with the gift of tongues, and as with any charismata, with the gift they are given comes much responsibility.
What do YOU think, kind reader?