Language converter is available on translated pages because of Title::getPageLanguage(), so it's possible to only translate messages in /zh and leave /zh-* alone. I propose adding zh-hans, zh-hant, zh-cn, zh-hk, zh-mo, zh-my, zh-sg, zh-tw to $wgTranslateBlacklist with a message like "Translate in zh please" to avoid duplicated manual work. (Where's the place to get local consensus if needed?)
By the way the difference between meta and translatewiki is that meta's translations are used for on-wiki display where the converter can be applied, while translatewiki's translations are exported to software strings...
See also [[m:Meta:Requests for bot status/Liangent-bot]]