OK, taking for granted that they were addressing God and praising Him, it was the Holy Spirit that was prompting them to speak, hence no interpretation was required. It was followed by Peter's message that was not in unknown tongue.
If we were to take a straw poll amongst a sizable number of rank-and-file Pentecostals and charismatics, I would imagine that a sizable percentage would still incorrectly believe that the Day of Pentecost, where the 120 were speaking in tongues, that the content of what they were saying was an evangelistic message initiated by the Spirit to the nearby Jews.
For the 120 themselves, we don’t know if they understood on the day that the sole purpose of tongues was as a prayer language between the individual and God or not, though we would expect that Jesus would have discussed this with the Disciples some time previously.
Going from the testimony of the nearby Jews, we know from Acts 2:11 that the 120 were
“declaring the wonders of God (NIV2)” which has its direct parallel with 1Cor 14:16
“…when you are praising God in the Spirit (NIV2)”. We find a second parallel between Acts 2:11 with 1Cor 14:2
“For anyone who speaks in a tongue does not speak to people but to God…(NIV2)” where this is exemplified with Acts 2:11 in that even though the Holy Spirit enabled the 120 (or at least many of them) to speak in known human languages, as the words that the Spirit was generating were being directed to the Father and not to man, for the Jews, they were understandably confused as they were not hearing a purposeful message being directed to them but fragmented outbursts of praise being directed to the Father.
If this were to
hypothetically occur during a congregational meeting (1Cor 14:2), where an unsaved visitor came in and heard someone or a number of people speaking of the praises of God in his native tongue, if there was no one else in the meeting who could understand what the visitor was hearing, then he would be just (or even more confused) as the Jews were. Whereas the Jews at least had the advantage with their religious upbringing, the unsaved visitor would be at a loss when he heard someone speaking the praises of God in his own language.
Now the visitor would not know if the person was a native of his own country but if he suspected that this was not the case, all that he could do would be to ask the person who was speaking why he was saying what he had, where the only reply could be, “undoubtedly I was speaking praises to God”. Even though this is only a hypothetical situation, if the Believer had the presence of mind to explain that it was the Holy Spirit speaking through him in praises to God, he could then (hypothetically) follow this up with an evangelistic word just as Peter had to do on the Day of Pentecost.
What was happening in Corinth after Paul left was totally a different story. It is like comparing an elephant to a dead mouse--possibly not even that!
Even though Paul’s primary concern with tongues in 1Cor 14 is with the uninterpreted use of tongues within the congregational setting, it does have a parallel with Acts 2 where even though the Jews could understand what was being said they were still totally confused. If those who understood the words being spoken were confused, how more confused would those be who had little or no understanding of the Gospel be. As Paul says, uninterpreted tongues will become a
negative sign that will only confuse and further isolate the unbeliever from the Gospel, which in effect becomes a
negative sign of judgment to the unbeliever, be they Jewish, Spanish, Italian or whatever.
Within the congregational setting, Paul is stressing that all our communications must be intelligible which is why he demands that every occurrence of tongues be immediately followed up with an intelligible articulation of the word of praise that someone had just offered to the Lord through the Holy Spirit.