Hi, the language is not a bar in upsc. You can given that exam in any language. In the preliminary exam the paper is generally printed in two languages one is Hindi and other is English .

Where as in the mains exam there are two compulsory papers one for English and other for one language that are mentioned in the constitution schedule 8. You have to qualify both the papers. And in the case of other paper you can choose your language of the examination like Hindi , English or any other languages for writing GS mains and optional and essay papers.

yes you can given both mains and prelims in English. Hope this helps.