In your preferences prefer zh-tw.

Now browse e.g.,
http://zh.wikipedia.org/wiki/Help:%E5%A6%82%E4%BD%95%E8%AE%BF%E9%97%AE%E7%BB%B4%E5%9F%BA%E7%99%BE%E7%A7%91

Yes, it looks like it is all zh-tw.

But now turn off stylesheets, etc. and look more closely at the top of
the page.

Help:如何訪問維基百科
維基百科，自由的百科全書
跳转到： 导航, 搜尋 <------do you see this line?
That line is still written in zh-cn!

If you can't see that line it is because you didn't turn off
stylesheets etc. in your browser.

And if we click on that line, we are brought to a header "導航" below,
which on the other hand is correctly in zh-tw.

Version: 1.12.x
Severity: enhancement