Use-case: As a user who is trying to find a page(s), I want to use the Category system.
If I have my wiki-Preferences set to a non-default language, the current system will only list pages which have been translated.
If the pages in my set-language are not at least partially translated, they will not appear within the category system that I am directed to, even if those pages are translated into one of the Fallback languages.

E.g.
https://meta.wikimedia.org/wiki/Category:Abstract_Wikipedia and
https://meta.wikimedia.org/wiki/Category:Abstract_Wikipedia/de
versus
https://meta.wikimedia.org/wiki/Category:Abstract_Wikipedia/en-gb and
https://meta.wikimedia.org/wiki/Category:Abstract_Wikipedia/de-formal