https://lawmin.nic.in/coi/coiason29july08.pdf
343. (1) The official language of the Union shall be
Hindi in Devanagari scriptt.
The form of numerals to be used for the official
purposes of the Union shall be the international form of
Indian numerals.
(2) Notwithstanding anything in clause (1), for a
period of fifteen years from the commencement of this
Constitution, the English language shall continue to be
used for all the official purposes of the Union for which
it was being used immediately before such
commencement:
Provided that the President may, during the said
period, by order1 authorise the use of the Hindi language
in addition to the English language and of the Devanagari
form of numerals in addition to the international form of
Indian numerals for any of the official purposes of the
Union.
(3) Notwithstanding anything in this article,
Parliament may by law provide for the use, after the said
period of fifteen years, of—
a) the English language, or
b) the Devanagari form of numerals,
for such purposes as may be specified in the law.