عَنِ الْهَرَوِيِّ قَالَ كَانَ الرِّضَاؑ يُكَلِّمُ النَّاسَ بِلُغَاتِهِمْ وَ كَانَ وَ اللهِ اَفْصَحَ النَّاسِ وَ اَعْلَمَهُمْ بِكُلِّ لِسَانٍ وَ لُغَةٍ فَقُلْتُ لَهُ يَوْماً يَا ابْنَ رَسُولِ اللهِ اِنِّي لَاَعْجَبُ مِنْ مَعْرِفَتِكَ بِهَذِهِ اللُّغَاتِ عَلَى اخْتِلَافِهَا فَقَالَ يَا اَبَا الصَّلْتِ اَنَا حُجَّةُ اللهِ عَلَى خَلْقِهِ وَ مَا كَانَ لِيَتَّخِذَ حُجَّةً عَلَى قَوْمٍ وَ هُوَ لَا يَعْرِفُ لُغَاتِهِمْ اَ وَ مَا بَلَغَكَ قَوْلُ اَمِيرِ الْمُؤْمِنِينَؑ اُوتِينَا فَصْلَ الْخِطَابِ فَهَلْ فَصْلُ الْخِطَابِ اِلَّا مَعْرِفَةُ اللُّغَات
Al-Harawi reports, “(Imam) al-Reza (peace be upon him) used to converse with the people in their languages. By Allah! He (peace be upon him) was the most eloquent and most knowledgeable of the people in their languages and mother-tongues.
One day, I asked him (peace be upon him), ‘O son of Allah’s Messenger! I am truly amazed at your knowledge of all these languages despite their variations!’
He (peace be upon him) replied,
‘O Aba al-Sult! I am Allah’s Proof upon His creation and it does not behove that He appoints somebody as a divine proof upon a nation while he does not know their languages. Did you not hear the saying of Ameer al-Momineen (peace be upon him), ‘We have been given the decisive statement?’ So, is the decisive statement anything except the knowledge of the languages?”
Notes:
1) Knowing the peoples’ languages is one of the signs of a divinely appointed Imam (peace be upon him). By this, one can easily distinguish a true Imam from a pseudo one.
2) Not only he knows all the languages but he is more eloquent than the native speakers.
3) The usurpers of caliphate could not even understand the language which was revealed in their mother tongue viz. Arabic. So, there is no question of them understanding other languages.
References:
1. Behaar al-Anwaar, vol. 26, p. 190, H. 1 and vol. 49, p. 87, H. 3
2. Oyoon Akhbaar al-Reza (peace be upon him), vol. 2, p. 228, H. 3