Any language is fine, the catch is not in language but Knowledge. Now Knowledge here has two aspects.
1. The knowledge that you are able to deliver in examination.
2. The Knowledge that you source.
Regarding 1. It depends on the language you are comfortable in, if by instinct for any question you think the answer in Hindi and then convert it to english and reply it is better you take examination Hindi (or any other language) because in mains you will not have the time to convert and it will decrease your efficiency as well as quality of your answers.
Regarding 2. If you are studying in 2/3 tier city and there are no english coaching institutes and you read everything is say hindi(gaining knowledge) and thinking of writing examination in english then also it is going to decrease your efficiency and quality because in IAS examination you need to understand and use exact words.
for example if you read in hindi regarding removal of president and convert it in english as word”removal” the it is wrong because word to be used is “Impeach”.
hence, you need to measure everything and choose wisely.