About legality on self-plagiarism when publishing an article
I don't know of any legal regimen in which self plagiarism is an issue. It is scientific misconduct, but not illegal, to use your own words and ideas without citing them. This is a case in which the legal and the ethical aren't the same. 

It isn't "stealing" ideas when you self plagiarize. It is just sloppy scholarship that makes it harder for readers and other scholars to find the complete context of the ideas expressed. 

Of course, if you have given up copyright to some words, then using those words again may violate the copyright that you once held, but no longer do. A publisher that holds the copyright might make a legal claim against you, but it isn't because of self-plagiarism, but copyright violation.