To reproduce:

• Assign a group that does not thave the edituserjs, editusercss, or the editusercssjs right the pagelang right.
• Change the page language for a CSS or js user subpage that does not belong to the current user. The action will succeed, even though the user had no right to modify the subpage in the first place.

Expectation:

• An error such as displayed on Special:Editcontentmodel would appear.

Environment:

• Vagrant 1.9.2 on Windows with MediaWiki 1.29-alpha (master, cloned today)

On investigation for another bug I noticed that Special:PageLanguage does not check if the user actually can modify the page language for the title, it just does so anyway. If I compare this to Special:Editcontentmodel, that special page actually checks if the user can change the content model for the page, which is why it displays an error.

I'm also wondering why the content language of a CSS/js user subpage can be changed, but that's not relevant to this bug.