MediaWiki now displays the language of the interface as the same the content by default.
For example, when you go to en.wikipedia.org, you get english interface by default and you get japanese interface for jp.wikipedia.org by default. You have to register an account and login to select the language of the interface to display.
That's not a big problem with most of the large languages, because the readers would like to read the language of the content, and they might be fine to read the language in the interface too.
However, there is an exception for Chinese. Chinese (zh) have two variants of writing styles, one is simplified Chinese characters (zh-Hans) used in mainland China, and the other one is traditional Chinese characters (zh-Hant) used in Hong Kong and Taiwan. zh uses zh-Hans for default. People have to get some additional training or learning sessions besides their normal education to read the other variant.
Now there is only one site for Chinese in Wikipedia (the major case of MediaWiki), that is zh.wikipedia.org. The content of zh site is mixed with simplified and traditional Chinese, and the site now uses a conversion program to convert the whole content into one variant that they could read easily.
So the problem is: traditional Chinese users see a simplified Chinese by default, and they have to register an account and choose the language of the interface at some place they might not even know to read their native language in the whole page.
I guess this default behavior might impact some other languages which have two or more different writing systems which share the same words.