The site language to be Chinese.
The site language is Chinese Singapore instead of Chinese.
as shown in the screenshot the value attribute sent from the select element is the same for both Chinese and Chinese Singapore (zh).
I just checked this, and yes, the entry for Chinese (Singapore) is incorrect (and has the same HTTP code as Chinese). We will fix this in a future release.
To get your site running right now, you can edit the language in the Language Manager (to be found at /wp-admin/network/settings.php?page=language-manager). Find Chinese (Singapore), edit the HTTP code to be zh-SG, and save the change.
Ok, thank you
Has been fixed for master in 49d2d92.
Fix HTTP code of Chinese (Singapore). #232
Closed with 43f64b1.