Navaros explained it quite well.
The Apostles were said to have been given a gift to speak (and more likely be heard) in all languages of the earth, so that when they preached, all races would understand their message at the same time.
This was clearly an advantage in the very mixed cultures of the Middle East at the time. It is of course, counted as a miracle of the Holy Spirit.
There are modern Christian sects that claim to have the same power, or individuals that enter a trance like state wherein they "speak in tongues".
To my knowledge, listening to them gives one the distinct impression that they are babbling uncontrollably and are not communicating in any language, let alone many distinct ones.
I suspect the experience is much along the same lines as trance states in many traditions, which are heightened emotional states that affect the individual and onlookers of the same mind.
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