Mottahedeh's reference to "a flexible exterior and private interior" a relevant in two spheres: the entire Iranian culture as well as individuals and their everyday experience.
First, the Iranian culture's "flexible exterior and private interior" relates to the continuity of their ancient culture and language even throughout the Arab conquests. The Egyptians and Mesopotamians lost nearly all of the thousands of years of their cultural identity after the Arab Islamic conquest. The "flexible exterior" of Iran has to do with the fact that it did not matter to the Iranians that the Arabs had taken over their territory; they had immense pride and passion for their culture and language and continued on with their tradition. They continued to speak Persian and keep with their tradition even though their culture was hardly mentioned in the Koran. The flexibility of Iran was that their culture and language prevailed while their external governing bodies varied greatly (Mottahedeh 164).
The "private interior" Mottahedeh refers to the ability of Persian speakers to keep an "emotional distance from the Turk and Mongol" peoples who ruled them. Their culture and language was a private recourse that no ruler could penetrate. Generations of harsh rulers dominated their territory, yet the Iranian's Persian traditions prevailed even through the worst of times (Mottahedeh 164-165).
This ambiguity is captured in the art of Persian poetry. In any notable Persian poetry there is not simply a translation that can bring the meaning from the private sphere to the public sphere.