Pashtuns were originally from the Sulaiman mountains and Waziristan located in today’s Pakistan. They expanded into Afghanistan and colonized many East Iranics, Dards and Nuristanis. Academics have referred to this Pashtun colonialism as “Pashtunisation”.
Safis were native inhabitants of eastern Afghanistan and they spoke a language known as Pashayi which is classified by linguistics as Indic language(descending from Sanskrit). After conversion, the Safi changed their language. They speak Pashtun now and are counted as Pathans.
Even today, a Kalash converting to Islam changes his language from Kalasha-Mun to Khowari(chitrali). Conversion leads to language change because certain languages are identified as “kafir” languages especially when a majority of its speakers have a non-Muslim origin.