Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
What you have mentioned in your question about al-Mahdi is not correct. Mahdi as stated in Sunnah is a man belonging to the family of the Prophet Muhammad (Blessings and peace of Allah be upon him). His name is the same as the name of the Prophet Muhammad (Blessings and peace of Allah be upon him) and his father's name is the same as of the Prophet Muhammad (Blessings and peace of Allah be upon him) father's name. Then his name is Muhammad or Ahmad son of Abdullah. He will be from the progeny of Fatimah the daughter of the Messenger of Allah. He will rule for seven years and fill the earth with justice. He will be Caliph for all believers, Arabs or non-Arabs. But the majority of the fighters who will fight in big wars in last days will be non-Arabs. It is narrated in Sahih Muslim. "the Romans will say: Do not stand between us and those (Muslims) who took prisoners from among us. Let us fight them. The Muslims will say: Nay, by Allah, we shall never turn aside from you and from our brethren so that you may fight them".
Imam Muslim narrated about the conquest of Constantinople that the Prophet Muhammad (Blessings and peace of Allah be upon him) said: " The Last Hour will not come until seventy thousand people from Bani Isaac (Is'haaq) attack it ".(Constantinople)
Imam Ibn Kathir believed that Romans will embrace Islam in the Last days, probably the conquest of Constantinople would be by their hands. There is no difference between an Arab and a non-Arab and between a white and a black except on the basis of piety. Allah Says in this concern (Interpretation of meaning): (Verily, the Most honourable of you with Allah is that who has At-Taqwa (God's fear). [49:13].
Allah knows best.