To my understanding, by convention, plone/app/locales/locales/zh_CN is the folder, zh-cn is the Language-Code. zh-CN is a code name by mistake, no difference from zh_CN. We might have the plone/app/locales/locales/zh folder with X-Is-Fallback-For: zh_TW ... something like that. However, that's not the case for the time being. For @maniachhz to see GUI with Chinese messages, one easy way is to set it as Traditional Chinese (繁體中文).
For Plone 5.0.4, you will see the UI like this:
If you set it as Chinese (中文) without country-specific language variants, very likely it remains showing (fallback) English UI.
Thanks for the ping. Hope this helpful.