Yes dear Student you Can write UPSC mains Examination in your regional language kannada because kannada is one of the language accepted by UPSC.But the question paper is printed in either in English or in Hindi unless you are answering the Compulsory language paper. It will be more difficult to write paper in kannada so I suggest you to write in English or Hindi.Here I am providing you link in which all the details about UPSC and its languages. https://competition.careers360.com/exams/upsc-civil-services-exam

Hope you understand this very Well and All the Best..