I do not think citizenship of Nepal is required. Only thing required is your birth certificate which is not equivalent to citizenship certificate.

I never had to submit or translate the citizenship certificate. All I submited are birth certificate from Municipality in their letter head (I had this done in English), and I also submitted the SLC's character certificate as a back up for that birth certificate which also has my DOB in vikram sambat. Based on those documents and without any RFE, I received my GC.

I am just sharing my experience. All you need is birth certificate. If you have original birth certificate, that is fine. If you don't have one, have it done from your VDC or Municipality. I suggest to get it done in English so you don't need to translate it back. You can ask for the letter pad in advance and have it printed in English and get their signature. But make sure that you have both of your parents' name on the birth certificate.