The current wikitext editor declares unnecessary lang HTML attribute on its internal source editor. For example, on MediaWiki, it’s en regardless of your language setting; on Mandarin Wikipedia, it’s zh regardless of your variant setting.
This seemingly harmless phrase actually breaks the Mandarin Wikipedia – CJK fonts require proper lang attribute to determine which set of characters and punctuation rules to render, which is why Mandarin Wikipedia has a very delicate system on distinguishing and converting, say, zh-Hans-MY and zh-Hant-TW.
The current behavior overrides the root <html lang> setting and makes the editor a pain to read. It should either copy the attribute from root <html> element or just remove this redundancy altogether.