UPSC rule on mixing languages:

The Civil Services Examination Rules state that candidates must answer each paper in the declared medium. Mixing languages within the same answer booklet is not permitted and can result in:

• Answers being ignored or marked zero
• Overall disqualification risk in extreme cases

Parenthetical technique (permitted and advisable):

You MAY write a technical English term in parentheses when writing in Hindi or another medium. For example:

• 'राजकोषीय घाटा (Fiscal Deficit)' in a Hindi-medium answer is acceptable
• This is a documentation aid, not 'mixing mediums'
• Keep it to genuinely technical terms — do not write entire sentences in English within a Hindi answer

Can different GS papers use different mediums?

Technically, the rules specify a single medium for the entire Mains examination (not per paper). Some past UPSC circulars have been interpreted as allowing per-paper choice, but UPSC's standard guidance is: declare one medium for all papers. Changing between GS1, GS2, GS3, GS4, and Essay is not recommended and may be flagged.

Exception — Literature optionals: If you chose Hindi Literature as optional, you write that optional in Hindi even if your main medium is English. Paper A is written in the Indian language you chose for Paper A. These are separate from your 'main medium'.

Source: UPSC CSE Rules (published in Gazette of India annually); UPSC FAQ section on upsc.gov.in