Based on my experience of working in the court in the US, you can translate Nepali docs with any Nepal Notary Public Council registered person. So, you need to need to go to Kitaab Khana in Babaarmal if that is far from you. http://notarypublic.org.np/renewed-translators-2/ these are the authorized people, and their phone number can be obtained from a Google search. No matter what you do, make sure the translator writes a letter of certification, that specifically has two sentences.

I certify the following statements "1. I am fluent and competent to translate Nepali and English language. 2. This is a complete and accurate translation of a Nepali Marriage Certificate into English"

I court, I have seen Kitab khana's translation being rejected by judges for not having those sentences.