It has been mentioned in the Hadith that Imam Mahdi alayhissalaam will speak with a stutter. He will speak with a slight stutter and occasionally this stutter will frustrate him causing him to hit his hand upon his thigh. I think this was mean but somehow lost in translation. He is 'Arab so will speak Arabic well. This proves that he is 'Arab and Ahl Bayt The Prophet (PBUH and HF) said: "al-Mahdi is one of us, the members of the household (Ahlul-Bayt)." Reference: Sunan Ibn Majah, v2, Tradition #4085 Hadhrat Umme Salmah (R.A.) narrates that Rasulullah (Sallallahu Alayhi Wasallam) said, " After the death of a Ruler there will be some dispute between the people. At that time a citizen of Madina will flee (from Madinah) and go to Makkah. While in Makkah, certain people will approach him between Hajrul Aswad and Maqaame Ibraheem, and forcefully pledge their allegiance to him.