It would be really great if the user interface is something that can be set in user preferences in other projects than just zh: ; it would make a lot of difference in data driven projects like WikiData, WikiSpecies and with a database backend Wiktionary.
It is not specific to zh. But the zh site supports multiple variants in the content, which is uniq atm.
I would prefer to have this only for registered users because it will prevent people changing stuff to their local language (not a good idea in the datadriven projects). When registering, the user would be more aware of what the project does and this would prevent a lot of these errors.
I think there is internal measurements to make sure only sysop can modify the mediawiki namespace. The UI switching itself is per user, so you and I can use different UI languages at the same site, for example.