The challenge: create an account on http://zh.wikipedia.org/ , change
the language (介面語言) to English in your preferences, then hope to see the
navigation of pages in English. You know, the same effect as uselang=en.

It won't work.
You'll have to logout first even if you want to see e.g.,
http://zh.wikipedia.org/w/index.php?title=User:Jidanni&uselang=en
in English mode.

Purging won't help. P.S. my 字體變換 choice was zh-tw (no "en" to
choose from there.)

Tried logging out/back in. Nothing cached at my end too.

Version: 1.11.x
Severity: normal