Gujarati Wikipedia (for example) has not all categories like English Wikipedia. If user is translating article about French Actor, the "French Actors" category won't be adapted as it isn't available in Gujarati Wikipedia. But, parent category of French Actors, Actors is available.
This was requested in this comment from Gujarati Wikipedia
Considering that version 2 of Content Translation allows users to add any category (T188615), we can extend the auto-complete suggestions to show parent categories for those categories that could not be adapted (only if those parent categories have a corresponding category in the target wiki and it has not been already added to the article).
This process will be done only for one level (i.e., not recursively) to keep things simple. So in the case that parent categories cannot be adapted, we won't explore their parent categories further.
With this approach, it is easy for users to enrich the classification of their translations while still keeping the control on what gets added automatically and why.