This was a highly theological occasion as it was a reversal of the curse of Babel and thus a demonstration of the dawning of the New Age. On the other occasions of tongue speaking, it would seem they spoke in a similarly strange way, but in their own language. When Cornelius and his group spoke in tongues, Peter and his group understood what they were saying. Content Peter in his sermon quotes Joel to explain the phenomenon witnessed by the crowd. Thus they spoke of God's Kingdom as it relates to us now and in the future.
Biblical prophecy is similar to exhortation.
It has a predictive strand, ie. Clearly tongues is not straightforward prophecy. The Joel reference puts it in the framework of dreams and visions, ie. The listeners were mystified by the messages, and only responded after hearing Peter's sermon. The imagery used here refers to God's judgment upon the nations.