emmm, I tried using zh-tw and zh-cn here, and it seems that we also need to create these pages for regional variants (they are even missing another message mw-sandbox.) It doesn‘t make sense to me since my wiki works fine without creating regional variants messages for sidebar. Sorry for the inconvenience (I suffered from creating 8 pages for one message a lot of time, because some of the other messages can’t fallback correctly, but I didn’t realize that a multilingual wiki like MediaWiki.org cannot even correctly fallback the sidebar.)
so:
/zh-cn, /zh-sg, /zh-my should use /zh-hans;
/zh-tw, /zh-hk, /zh-mo should use /zh-hant.
/cdo doesn’t make sense to me; it seems like Vietnamese related.
mw-sandbox
/zh-hans, /zh-hant, /zh-cn, /zh-tw, /zh-hk, /zh-mo, /zh-my, /zh-sg: 沙盒
Sorry again and thanks for your help! that 12 pages is now correctly loaded and seems perfectly fine.