Here’s what the draft constitution says about the right of a majority ethnic group in a given province to use their own mother tongue as official language in that province:
Paragraph 6. Language of the nation: All the mother tongues spoken in Nepal shall be the national language.
Paragraph 7. Language of official transaction: (1) The Nepali language written in Devnagiri script shall be the language of official business in Nepal.
(2) In addition to Nepali language, a province may select one or more national language that is spoken by majority of people in that province as the language of official business, as provided for by law.