That's not entirely correct. Hindi is the official language of Republic of India. That is why 14 September is celebrated as Hindi Divas: https://en.wikipedia.org/wiki/Hindi_Divas
That is not entirely correct either. Hindi and English are official languages of the Republic of India. If you watched the swearing-in ceremony of the recent Modi government some cabinet members chose to be sworn-in in English.
There is a difference between "national" and "official".