nit: Maybe just call it `Unable to find components in categories "%s"!` to make this more clear?

No declarations after statements please, we (still) need to be compatible with older C versions/compilers.

nit: Maybe better: `List components that are part of the specified categories (listed separated by commas).`

Translator hint needs update.

I'm not sure about the naming here... It would make sense to implement this as a search filter, to search by category in `appstreamcli search`, but if this stays the way it is something like `category-contents` might make more sense: `appstreamcli category-contents AudioVideo` or `appstreamcli search --by-category=AudioVideo mysearchterm`.