Part XVII of the constitution:
This part 17, defines an OFFICIAL language, NOT a national language.
Article 345: This gives the State govt., power to decide its own "OFFICIAL LANGUAGE"
Article 343: This defines Hindi in devangari script and English to be the "OFFICIAL LANGUAGES" of union govt.
DIFFERENCE between National and Official Language:
NATIONAL LANGUAGE: Defines the people of the nation, culture, history.
OFFICIAL LANGUAGE: A language that is used for official communication
While a National language by default can become the Official language, an Official language has to be APPROVED legally to become the National language.
All languages spoken in India, starting from the most populous to the least are our national languages, because all of them define the people of this nation, culture and their history collectively.