At http://zh.wikipedia.org/ you'll notice three additional tabs at the right-hand of the top tab bar: "不转换" for unconverted display, "简体" for simplified conversion, and "繁体" for traditional conversion. These conversions don't change the source text or alter editing; it's for rendered page display and user interface only.
The implementation also supports a markup that allows manually specifying the conversion of a specific word/phrase in situations where the mapping table is not sufficient. For example, -{zh-cn:foo; zh-tw:bar}- will show up as "foo" when "简体"(Simplified Chinese) is selected, and "bar" when "繁体"(Traditional Chinese) is selected; -{foo}- will show up as "foo" no matter what language variant is selected (this should be sufficient for things like quotes, math, etc). There is also a way of customizing the mapping table through a page in the MediaWiki: namespace.