Translate SLC certificate in english and put nepali (and english in bracket). You can also put a BS to AD date conversion website screenshot. Also see if you have older certificates like 5th grade, 8th grade district exam certificate with DOB. TBA bhaneko susare or home birth ho, I think. So, ask someone who has knowledge of your birth or who was present at that time to give affidavit. This worked in my sister's case.