As how I understand it, as long as the input effort of writing a product (book/thesis/paper) comes from a person, then that one will be the author. No matter how hard their critics or useful their suggestions are, the editors/advisor(s)/reviewers are not co-authors. But once they actually roll up their sleeves to verify the ideas (not just providing ideas), then they should be considered as co-authors, regardless of how small their parts appear in the final works.
Things get complicated in translated works. Since the works are purely about language, giving ideas is verifying them. So technically, anybody who involves in the process should be called as co-translators, even though we can borrow the term “editor” to call people who check the shaped works in later stages of the process. But if the changes made by any of them in whatever stage are sufficiently large to reshape the previous version, then they should be called as co-translators, not editors anymore.
Is my understanding correct? I post it here because this community may have more experience in authorship (or translatorship?). My interest is on theoretical fields, if necessary.