The first part of your question is answered in question 2945 (website: 3174). Please refer to the website in order to see the appropriate response. In regards to the second part of your question, if you mean to say that the language of the inhabitants of hell is Farsi, it should be understood that there are no authentic narrations which would support this claim. As a matter of fact, our observations seem to contradict this claim because many of our religious leaders and figureheads communicate through means of this language. However, if you mean to say ‘jahannam’ is in origin a Farsi word, it is possible that this may be correct, as this claim is supported in some dictionaries. However, the potential verity of this statement does not hint at a flaw within the Farsi language. Conversely, other words (with positive connotations) such as ‘ferdos’ meaning paradise and ‘estabraq’ which is a kind of fabric found in paradise are both words that have been mentioned in the Quran and whose roots reside in the Farsi language.
The first part of your question is answered in question 2945 (website: 3174). Please refer to the website in order to see the appropriate response. The second part of your question can be viewed from one of two perspectives below:
1. Do the inhabitants of hell communicate through means of the Farsi language?
2. Is the word ‘jahannam’ a Farsi word?
We will briefly address each of the questions above:
1. In regards to the first question, although it is mentioned in some narrations that Arabic is the language spoken by the inhabitants of heaven, this does not in any way imply that Farsi is the language spoken by those in hell. In the case that a narration does exist in this regard, they would be rendered unauthentic for the following reasons:
a) It has been narrated that in the presence of Salman Farsi, the Prophet (pbuh) stated: The people of Fars (Iran) would acquire iman (faith) even if it were hanging from the star of Pleiades.
b) Abu Hashim Ja’fari narrates: While sitting at the presence of Imam Hadi (as), I noticed that he was speaking Farsi and Saqlabi (a language spoken by a tribe from present day Caucasia) with some of his servants.
c) An individual from Khurasan (a province within Iran) came to the tenth Imam (as) and spoke with him in Arabic. To the Iranian’s surprise, the Imam responded to him in Farsi.
d) It is narrated that Prophet Sulaiman (as), who was well acquainted with a host of languages (even that of the birds), would speak in Farsi at times of war.
e) A group of Iranians were at the presence of Imam Sadiq (as) as he was delivering a sermon. They said amongst themselves, “We are unable to understand what the Imam is saying because we are unfamiliar with the Arabic language”. Upon hearing this, the Imam repeated his words of advice in Farsi and said “هرکه درم اندوزد، جزایش دوزخ باشد”.
The fact that many of Islam’s religious leaders speak in this language and the immense amount of respect that has been paid to Iranians by individuals such as Imam Ali (as) – to the extent that he was often criticized by some prejudiced and intolerant people – clearly indicates that this language could not possibly be one which is spoken by the inhabitants of hell. More importantly, it should be understood that Iranian Muslims have never been prompted to desert their language for another.
2. As to the whether or not ‘jahannam’ is a Farsi word, it is mentioned in some dictionaries that it is derived from the Farsi word ‘jahnam’ which literally means a deep well.
Needless to say, if this is true, it does not negatively impact the Farsi language by any means. There are a myriad of words in Arabic that find their roots in various languages (i.e. Farsi). For example, the word ‘ferdos’, meaning paradise, has been used in the Quran and finds its origin in the Farsi word “pardis”. Examples of other words can be found within this Holy Book, such as “sundus” and “estabraq”, which are not of Arabic origin, but take root in other languages such as Farsi.
 Muhammad Baqir Majlisi, Biharul-Anwar, vol. 11, pg. 56, hadith 57.
 Ibid, vol. 22, pg. 52, hadith 2.
 Ibid, vol. 49, pg. 87, hadith 2.
 Muhammad ibn Yaqub Kuleini, Kafii, vol. 1, pg. 285.
 Biharul-Anwar, vol. 14, pg. 110, hadith 2.
 Ibid, vol. 47, pg. 119, hadith 162.
 Ibn Abil-Hadid, Sharh Nahjil-Balaghah, vol. 19, pg. 124.
 Ibn Manthur, Lisanul-Arab, vol. 12, pg. 112.
 Mu’minun:11 “الَّذینَ یَرِثُونَ الْفِرْدَوْسَ هُمْ فیها خالِدُون”; Kahf:107 “إِنَّ الَّذینَ آمَنُوا وَ عَمِلُوا الصَّالِحاتِ کانَتْ لَهُمْ جَنَّاتُ الْفِرْدَوْسِ نُزُلا”
 Kahf:31 “یَلْبَسُونَ ثِیاباً خُضْراً مِنْ سُنْدُسٍ وَ إِسْتَبْرَق”
 Lisanul-Arab, vol. 10, pg. 5.