Who will justify the man that sins against himself?: This is a rhetorical question, not a request for information. It can be translated as a statement, as Good News Translation does. No one can justify (or, excuse) a person who is not fair with himself. Good News Translation says “There is no excuse for a person to run himself down.” “Run himself down” is the equivalent to sins against himself. It is an English idiom for speaking badly of oneself, thinking of oneself as worthless, having low self-esteem. So we may translate “There is no excuse for a person speaking badly of himself” or “Who can excuse a person who thinks of himself as worthless?” Man here has the sense of “person.” This is also true in verses 30-31.

And who will honor the man that dishonors his own life?: This is essentially a repetition of the thought in the first line. Good News Translation offers a good translation, again as a statement rather than a question.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Sirach. (UBS Helps for Translators). New York: UBS, 2008. For this and other handbooks for translators see here.