Not strictly a bug, but more of an open question:
The "about this article" section is currently shown using the System language instead of the article language. If you change your system language, these strings will appear in the selected language. Let's take a minute during grooming to think about whether we want to override these strings using the article language.

I see. Thanks, @Dbrant.
In my opinion, the "About this article" looks like a part of the article instead of an app-level interface, and that's the reason why I did not have a thought to change the system language to see the translated strings. Yes, we can talk about this during the Grooming.