Question: How will the people be addressed on the Day of Resurrection? And will Allaah, the Most High, address the people with the language of the Arab? And is it correct that the language of the People of Fire is Farsee (Persian) and the language of the People of Paradise is Arabic?
Answer: All praise is due to Allaah, the Lord of all that exists. I do not know with what language the people will speak on that day! And I do not know how they will hear the Speech of their Lord, Jalla wa 'Alaa, because Allaah, the Most High, has not informed us about that. And nor has His Messenger, may the blessings and peace be upon him.
And it is not correct (to believe) that Farsee is the language of the dwellers of Jahannaam and nor (is it correct) that Arabic is the language of the people of eternal bliss. And we do not know of any dispute regarding that between the Companions (RadhiyAllaahu 'Anhum). Rather, they were all sufficed with that (the knowledge they had). Because speech which is similar to this is the speech of curiousity except it was newly invented by the people who came afterwards (the khalifiyoon, from the latter generations).
So the people said, "They will address (each other) in Arabic".
And some others said, "So the People of the Fire, they will respond to Farsee, and this is their language in the fire."
And some others said, "They will converse in Siriyaniyyah, because it is the language of Adam, and all (other) languages branched off from that."
And some others said, "So the People of Paradise, they will speak in Arabic."
All of this speech, the one's who possess it, they have no proof for it, from the intellect nor from the narrations. Rather they are claims devoid of any evidences. And Allaah knows best and is the Most Wise.
Source: Majmoo' al-Fataawa, Volume 4, Number 300