The word "nations" is a metonym for the people who live in the nations. These words can be translated in active form. AT: "People will curse whoever says to the wicked person, ... , and the people of other nations will hate him"
Possible meanings are 1) people should never call any wicked person a righteous person or 2) no one should say of a person guilty of a crime that he is innocent. AT: "a person guilty of a crime ... innocent"