I am a motivational speaker and am self publishing a nonfiction book about child abuse. i ahve some inappropriate language in the book and want to know if my designer by law can use a parental advisory sticker on my cover?
Depends on the country's laws and target-audience of the book. Books in general do not have a parental advisory on them as would a video-game or CD would.
Considering the sensitive topic of the book, you may wish to view articles on the reactions on Speak by Laurie Halse Anderson. A book that has language in it does not mean it's illegal, but it also does not prevent it from being proposed as ban-able by those offended by its publishing.
For more information check out the ALA: