Even having category redirects work properly (so that if [[Category:Foo]] redirects to [[Category:Bar]], putting an article in [[Category:Foo]] means it shows up in [[Category:Bar]] - much like redirected templates work) would be most helpful in allowing Commons to use languages other than English for its category tree - so that different-language names for the same thing would work the same, e.g. [[Category:Horse]], [[Category:Cheval]] and [[Category:Hauspferd]] could all point to [[Category:Equus caballus]] and Just Work.
- d.