Special:Badtitle/NS90:Talk:Translation Workflow/Language-selector under Toolbox in sidebar/reply (3)

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.