Rightly, after the Islamic Futuhat in Iraq and the Levant, Muslims under the leadership of Umar assigned Muslim governors to the new provinces, but the existing Byzantine and Persian administrators were retained wherever possible.  For fifty years, in fact, Greek remained the chancery language of Syria, Egypt, and Palestine, while Pahlavi, the chancery language of the Sassanians, continued to be used in Mesopotamia and Persia.( A Brief History of Islam)