cassman:
What purpose does this serve?
What if no one understands what the person is saying?
How is one to discern if it is “the real thing” or not?
Excellent question, and I thank you for asking it without any snide tone.

I am a Charismatic Catholic who does pray privately in tongues but does not prophecy in tongues (speak publicly in tongues). Tongues are only one of the gifts of the Holy Spirit, and are frankly one of the lesser gifts, but seem to be THE gift that many people outside of the Charismatic Renewal take great exception to, for whatever reason. The Charismatic Renewal, contrary to what many trasditionalists will say, is an approved lay apostolate of the Catholic Church and operates in full obedience to the Holy See. I’d be happy to provide the links here, if needed, though I have already posted them in about 6 other threads.
Ok, Tongues: Scripture seems to indicate that there is more than one “variety” of tongues. There is public prophesy with interpretation as said in 1 Cor.; there is speaking in another earthly language, as was done by the disciples in Acts; and there is the “groanings of the Spirit” or private prayer in tongues as in Romans 8:26. It would seem to me that the first two would need interpreters, and the third one does not.
Even in 1 Cor 14:28 it says “But if there is no one to interpret, there should be silence in the assembly, each one speaking only to himself and to God”. This says two things - first, that just because there is no one interpreting for humans does not mean that the tongues do not exist. Second, this says silence “in the assembly”, as in, not speaking in tongues so that all can hear. Rather, one is indicated here to keep “silent” – to keep it to themselves rather than speaking publicly. This then ties into Romans 8:26 (groanings of the Spirit) and shows that one could have the gift of “silent” tongues, only between the individual and God.
As for speaking in tongues, it is the Holy Spirit which speaks through us, to express things which are beyond words (those who speak in tongues often say that it happens when the words of praise/prayer no longer suffice). It can either be in Earthly (“foreign”) languages (which can sometimes be understood by their native speakers), or in the languages of the angels. Both of these could be interpreted through the gift of interpretation of tongues. As stated, this form of personal communication differs from the ones requiring interpretation.
Scott Hahn in his “Scripture Matters” explained that that revival of the gift of tongues are a warning from God (it also happened in the Old Testament before some disaster struck, and also in the New, before the Romans invaded). Yikes! That gives us something to think about, doesn’t it!?