What does it mean that they spoke with other tongues at Pentecost?
At Pentecost, speaking with other tongues meant speaking known human languages, not unknown guttural noise. The Holy Spirit gave the apostles the gift to speak the various languages of Jews gathered from Babylon, Spain, and elsewhere, so every person heard in a language they could understand.
Les explains that Pentecost was Jewish, not Gentile. When the day of Pentecost fully came, a sound like a rushing mighty wind filled the house, and cloven tongues like fire sat upon each one. They were all filled with the Holy Spirit and began to speak with other tongues. Les says these were other languages, not an unintelligible guttural noise, because Jews from all over had come from places like Babylon and Spain and spoke different languages. The miracle was that God gave these apostles the gift to speak all the languages of the people in that massive crowd, so every man heard them speak in a language he could understand. This was the first time in Scripture that the Holy Spirit manifested the gift of speaking known languages that were not intrinsic to the speakers' own education. Peter and most of the disciples were from Galilee, uneducated fishermen, yet suddenly they spoke the various languages evident at Pentecost.
And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
More Questions
What were the 'other tongues' spoken at Pentecost?
The 'other tongues' the disciples spoke at Pentecost were other real languages, not unknown speech.
Who received the gift of tongues the first and second times it happened in Acts?
In Acts the tongue-speaking occurrences follow a pattern: the first time it happened to Jews in Acts 2, strictly to the Jew, and the second time it happened to a whole house of Gentiles.