Hello Rana pratap singh,.                                                                                                                                          Yes, you can write your GS paper in English and essay paper in Hindi of UPSC  mains.                                                                                                                     Now UPSC provides options to write GS and Optional papers in different mediums (Since 2018). If you choose to write GS and Essay papers in Hindi and Optional Paper in English you can do it.

One thing that you should be remembered here is that if you write GS and essay in English then, you have to write optional in English. You can write GS and essay in your comfort language but you need to write in english as an optional

You will get these option while filling the application form for prelim. UPSC ask optional subject and medium both for GS & Essay papers and Optional subject separately in the preliminary stage itself. So, you get sufficient time to get yourself ready and mater in the chosen language.                                                                                                                       ALL THE BEST.