All students, irrespective of the medium of study, got selected in the top ranks of UPSC exams. So the medium of examination does not determine your rank. Only your knowledge determines your rank. So if you are not from the English medium, you have the same chance as that of an English medium student to crack UPSC examinations. But you should follow the below instructions.
All objective papers (Preliminary Examination) are in English and Hindi medium only. So you need to understand English or Hindi and answer.
But in descriptive papers (Mains Examination), questions are printed in English and Hindi. But you can answer in any of the scheduled languages mentioned in the Constitution. These are major regional languages such as Telugu, Tamil, Kannada, Malayalam, etc.
In the interview also, you can answer in any of the scheduled languages mentioned in the Constitution. You will be assisted by a Translator.
As of 2021, 22 languages are listed as scheduled languages in the Constitution. They are – Assamese, Bengali, Bodo, Dogri, Gujarati, Hindi, Kannada, Kashmiri, Konkani, Maithili, Malayalam, Manipuri, Marathi, Nepali, Odia, Punjabi, Sanskrit, Santali, Sindhi, Tamil, Telugu, and Urdu.
This holds true for all UPSC exams.