Just translate the section of the category would create a page/ca untranslated. I think the "solution" report a bug (wishlist) over mediawiki software.
You are right, off course, but it doesn't seem to be very much different from the situation when the translated page doesn't exist at all: In both cases the user will see the original English text. The only difference is that your language will appear in the language selection bar and that the page contains a translated category. How big a difference will this be to ordinary users?
(I am not exactly pleased with this proposal myself - it isn't really a tidy solution, but would anyone notice the difference?)