When you're not logged in, visit Minecraft Wiki (ZH) with this header part accept-language: zh-CN,zh;q=0.9 will cause a situation that system messages on the whole page displays in English, instead of the expected Simplified Chinese (zh-cn or zh-Hans-CN). This problem also occured on my own wiki, with MediaWiki version 1.35.1. Only system messages that contains a /en subpage are affected.

This problem can cause a severe readability issue on Chinese sites.
It's reported by Traditional Chinese users that this kind of header part accept-language: zh-TW,zh;q=0.9 doesn't run into this problem, and will correctly show system messages in Traditional Chinese.
Currently, there's a workaround that by manually changing (or creating) the /zh-hans or zh-cn subpages, thus idk whether it's really related to Language Converter.