Language-selector under Toolbox in sidebar
They are different, I want to change userbase's interface language, look at this snapshot http://is.gd/fFIAm.
The chinese category links have a lot of work need to do, I will modify chinese category link name when I have free time.
Ah - I thought you meant to the Translate language options. The interface settings are done from your Preferences screen, where zh-CN and zh-TW can be set. You can change them as often as you need to, it won't cause any problems.
If that still leaves you with things that need changing in the system messages I'll get someone with root access to join this thread.
Change Preference need an account and login in, but some visitors (guest) don't have (especially the person who visits this site first time).
another method is add this function to Translation Extension, for example, the extension not only redirect browser to /zh-cn page, but also change wiki' interface language（system messages) to zh-CN.