The Safavids by the time of their rise were Azerbaijani-speaking although they also used Persian as a second language. The language chiefly used by the Safavid court and military establishment was Azerbaijani. But the official language of the empire as well as the administrative language, language of correspondence, literature and historiography was Persian. The inscriptions on Safavid currency were also in Persian.
Scene from Attar's The Conference of the Birds, by Habibulla Meshedi (1600).
Safavids also used Persian as a cultural and administrative language throughout the empire and were bilingual in Persian. According to Arnold J. Toynbee, In the heyday of the Mughal, Safawi, and Ottoman regimes New Persian was being patronized as the language of litterae humaniores by the ruling element over the whole of this huge realm, while it was also being employed as the official language of administration in those two-thirds of its realm that lay within the Safawi and the Mughal frontiers
According to John R. Perry, In the 16th century, the Turcophone Safavid family of Ardabil in Azerbaijan, probably of Turkicized Iranian, origin, conquered Iran and established Turkic, the language of the court and the military, as a high-status vernacular and a widespread contact language, influencing spoken Persian, while written Persian, the language of high literature and civil administration, remained virtually unaffected in status and content.
According to Zabiollah Safa, In day-to-day affairs, the language chiefly used at the Safavid court and by the great military and political officers, as well as the religious dignitaries, was Turkish, not Persian; and the last class of persons wrote their religious works mainly in Arabic. Those who wrote in Persian were either lacking in proper tuition in this tongue, or wrote outside Iran and hence at a distance from centers where Persian was the accepted vernacular, endued with that vitality and susceptibility to skill in its use which a language can have only in places where it truly belongs.
Prince Muhammad-Beik of Georgia by Reza Abbasi (1620)
According to É. Á. Csató et al., A specific Turkic language was attested in Safavid Persia during the 16th and 17th centuries, a language that Europeans often called Persian Turkish ("Turc Agemi", "lingua turcica agemica"), which was a favourite language at the court and in the army because of the Turkic origins of the Safavid dynasty. The original name was just turki, and so a convenient name might be Turki-yi Acemi. This variety of Persian Turkish must have been also spoken in the Caucasian and Transcaucasian regions, which during the 16th century belonged to both the Ottomans and the Safavids, and were not fully integrated into the Safavid empire until 1606. Though that language might generally be identified as Middle Azerbaijanian, it's not yet possible to define exactly the limits of this language, both in linguistic and territorial respects. It was certainly not homogenous—maybe it was an Azerbaijanian-Ottoman mixed language, as Beltadze (1967:161) states for a translation of the gospels in Georgian script from the 18th century.
According to Rula Jurdi Abisaab, Although the Arabic language was still the medium for religious scholastic expression, it was precisely under the Safavids that hadith complications and doctrinal works of all sorts were being translated to Persian. The 'Amili (Lebanese scholars of Shi'i faith) operating through the Court-based religious posts, were forced to master the Persian language; their students translated their instructions into Persian. Persianization went hand in hand with the popularization of 'mainstream' Shi'i belief.
According to Cornelis Versteegh, The Safavid dynasty under Shah Ismail (961/1501) adopted Persian and the Shi'ite form of Islam as the national language and religion.
Legacy
It was the Safavids who made Iran the spiritual bastion of Shi'ism against the onslaughts of Sunni Islam, and the repository of Persian cultural traditions and self-awareness of Iranianhood, acting as a bridge to modern Iran. The founder of the dynasty, Shah Isma'il, adopted the title of "Persian Emperor" Pādišah-ī Īrān, with its implicit notion of an Iranian state stretching from Khorasan as far as Euphrates, and from the Oxus to the southern Territories of the Persian Gulf
