Special Provisions
(1) The Union's official language is Hindi in Devanagari script, using international Indian numerals for official purposes. (2) Notwithstanding this, English continued for all Union official purposes for 15 years from the Constitution's commencement, with the President able to additionally authorise Hindi/Devanagari numerals during that period. (3) After that 15-year period, Parliament may by law provide for continued English or Devanagari-numeral use for specified purposes.
Hindi is India's constitutionally-designated official language of the Union - but the Constitution itself built in a 15-year transition period (1950-1965) during which English would continue for official purposes, with Parliament given the power to extend English's official use further after that. In practice, Parliament exercised this power (via the Official Languages Act, 1963) to keep English in permanent, ongoing use alongside Hindi, rather than letting the original 15-year sunset actually take effect.
This is why English continues to be used for official Union government purposes today, decades past the original 1965 sunset date - Parliament used its Article 343(3) power to keep it in permanent use, reflecting the practical reality of India's linguistic diversity and non-Hindi-speaking regions' concerns.