The phenomenon might have been a mixture of both xenoglossy and glossolalia.
1When the day of Pentecost came, they were all together in one place. 2Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting.
The phenomenon began with sound and sight.
3They saw what seemed to be tongues of fire that separated and came to rest on each of them. 4All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
5Now there were staying in Jerusalem God-fearing Jews from every nation under heaven. 6When they heard this sound, a crowd came together in bewilderment, because each one heard their own language being spoken.
This was xenoglossy or xenolalia, i.e., a person is able to speak a language they could not have acquired by natural means.
7Utterly amazed, they asked: “Aren’t all these who are speaking Galileans? 8Then how is it that each of us hears them in our native language? 9Parthians, Medes and Elamites; residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, 10Phrygia and Pamphylia, Egypt and the parts of Libya near Cyrene; visitors from Rome 11(both Jews and converts to Judaism); Cretans and Arabs—we hear them declaring the wonders of God in our own tongues!” 12Amazed and perplexed, they asked one another, “What does this mean?”
Multiple persons practiced xenolalia and they spoke simultaneously. It was at least a mixture in xenolalia of different languages.
13 Some, however, made fun of them and said, “They have had too much wine.”
As a whole scene, it appeared somewhat like a drunken speech demonstration. Some observers were confused.
14Then Peter stood up with the Eleven, raised his voice and addressed the crowd: “Fellow Jews and all of you who live in Jerusalem, let me explain this to you; listen carefully to what I say. 15These people are not drunk, as you suppose. It’s only nine in the morning! 16No, this is what was spoken by the prophet Joel:
17“ ‘In the last days, God says,
I will pour out my Spirit on all people.
Your sons and daughters will prophesy,
Glossolalia could be involved as well.
your young men will see visions,
your old men will dream dreams.
From an individual's perspective, it was xenolalia. Taken as the whole group phenomenon, it appeared as glossolalia as well.