No, they do not have to be notarized. However, I read from the USCIS website, it must be from an individual that is competent in both languages. It must be signed by them and include date and address. I added this line at the bottom of my certified birth certificate:
I __________ certify that I am fluent (conversant) in the English and Spanish languages, and that the above/attached document is an accurate translation of the document attached entitled _________________________.