The Kushan Empire expanded out of what is now Afghanistan into the northwest of the subcontinent under the leadership of their first emperor, Kujula Kadphises, about the middle of the 1st century CE. They came of an Indo-European language speaking Central Asian tribe called the Yuezhi, a branch of which was known as the Kushans. By the time of his grandson, Kanishka, they had conquered most of northern India, at least as far as Saketa and Pataliputra, in the middle Ganges Valley, and probably as far as the Bay of Bengal.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What was the language base of the Kushans?
Indo-European