On http://zh.wikipedia.org/wiki/Special:Preferences
Sure one can pick
Language(介面語言): [zh-tw - ‪中文(台灣)‬]
But upon saving, it snaps back to "zh".
OK, set it to English, save, then set it to zh-tw. Seems to work, but
then when you reenter Preferences you'll see it has become zh again.

And if all that still works for you, then try setting both
Language:
Variant:
to zh-tw. When you click Save, you'll see they have snapped back to zh.
Browser used: Firefox, emacs-w3m, etc.

I get it, the most one can do is set
Language: zh-hant
Variant: zh-tw

OK, so you will probably change the above Product to MediaWiki, to rip out the now no longer valid choices so nobody
can click them by accident anymore.

But it so happens that I am also the owner of three $wgLanguageCode='zh-tw' wikis. What disaster will that spell for them?

Version: unspecified
Severity: normal
URL: http://zh.wikipedia.org/wiki/Special:Preferences