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You need to provide the copy of the death certificate. If you don't do the 公证, you may ask your friend to translate it into English and submit the copy of the original document and the translation version.  Read this link for Document Checklist, etc.:

https://www.uscis.gov/n-400

See these websites for the translation requirement by USCIS:

https://www.uscis.gov/ilink/docView/AFM/HTML/AFM/0-0-0-1/0-0-0-2061/0-0-0-2253.html

https://www.immihelp.com/translation-of-non-english-documents-for-us-immigration/