Speaking is a metonym for using words to defend innocent people. AT: "Defend those who cannot defend themselves" (See: [[rc://en/ta/man/translate/figs-idiom]])
The cause represents the person whose cause it is. AT: "so that people will treat all who are perishing justly" (See: [[rc://en/ta/man/translate/figs-metonymy]])
# for the causes
The ellipsis can be filled in. AT: "speak for the causes" or "speak out for" (See: [[rc://en/ta/man/translate/figs-ellipsis]])
# poor and needy people
These two words have basically the same meaning and are used together for emphasis. AT: "people who are poor and cannot get the things that they need" (See: [[rc://en/ta/man/translate/figs-doublet]])