Did the foreigners hear a voice in their heads automatically translating a heavenly tongue, or did they hear the disciples speaking in their own languages? Scriptures tell us plainly what the foreigners heard:
Acts 2:6 "And when this sound occurred, the crowd came together, and were bewildered because each one of them was hearing them speak in his own language."
It was the disciples who were speaking their languages, not a voice in their heads.
If that's not proof enough then look at verse 4. It says:
"they began to speak in other tongues as the Spirit enabled them".
The Greek word for tongues is glossa and any lexicon will tell you one of the meanings is 'languages'. The same word that appears in v11:
Acts 2:11 "Cretans and Arabs—we hear them in our own tongues speaking of the mighty deeds of God.”
In v11 the word tongues is clearly referring to human languages. So to say that "tongues" in v4 means a heavenly language would contradict its usage in v11 where it clearly means human languages.
Notice also the plural tongues in verse 4. The disciples spoke in multiple languages, not a single heavenly language.
Verse 4 also us what the Holy Spirit did, and who he did it to - the Holy Spirit enabled the disciples to speak in other tongues (languages). There is no mention of the Spirit falling on the unregenerate crowd and performing a miracle of hearing in their ears. If there was this great miracle of automatic translation in the ears of the crowd Luke would have told us, not remain silent on such an important fact.
So there can be absolutely no doubt that the disciples spoke in foreign human languages.
When they spoke in tongues there were thousands of foreigners present. They were not in a room inside a house. It could only have been in the grounds of the Temple Courts where thousands of Jewish pilgrims had gathered for the Feast of Pentecost.