Although Middle Persian was the native language of the Sasanians (who, however, were not originally from Pars), it was only a minority spoken-language in the vast Sasanian Empire; it only formed the majority of Pars, while it was widespread around Media and its surrounding regions.
Due to invasions from the Scythians and their sub-group, the Alans into Azerbaijan, Armenia, and other places in Caucasus, the places gained a larger, although small, Iranian population.