Yes, you can choose the language in which you want to write the UPSC Civil Services Examination. The UPSC provides the option to choose one of the scheduled languages listed in the Eighth Schedule of the Indian Constitution or English as the medium of examination. The languages available as options include Assamese, Bengali, Gujarati, Hindi, Kannada, Kashmiri, Konkani, Malayalam, Manipuri, Marathi, Nepali, Oriya, Punjabi, Sanskrit, Sindhi, Tamil, Telugu, Urdu, Bodo, Santhali, Maithili, and Dogri.
You need to select the language option at the time of filling out the application form for the examination. The selected language will be the medium for answering the papers in the Main Examination. However, the General Studies Paper (Paper 1) of the Preliminary Examination will be conducted in both English and Hindi.
It's important to note that the language chosen for the Main Examination will also determine the availability of study materials, books, and resources in that language. Therefore, consider your proficiency in the chosen language and the availability of relevant resources before making a decision.

Another

Yes, candidates can choose the language in which they want to write the UPSC Civil Services Examination. The examination is conducted in both English and Hindi languages. Additionally, candidates can also choose to write the exam in any of the 22 scheduled languages recognized by the Indian Constitution. The list of 22 scheduled languages includes Assamese, Bengali, Gujarati, Hindi, Kannada, Kashmiri, Konkani, Malayalam, Manipuri, Marathi, Nepali, Oriya, Punjabi, Sanskrit, Sindhi, Tamil, Telugu, Urdu, Bodo, Santhali, Maithili, and Dogri.
Candidates who choose to write the exam in any language other than English or Hindi will be given a bilingual question paper, i.e., the question paper will be in the selected language and in English. Additionally, candidates who opt for a regional language will also be given a regional language translation of the General Studies Paper-II (CSAT) which is qualifying in nature and only needs 33% marks to qualify.
It is important to note that candidates cannot switch the language medium once the registration process is completed. Therefore, candidates must choose their preferred language medium carefully during the application process.