Steps to reproduce:

• Visit zh.wikipedia.org from a browser with no explicit accept-language preferences for Chinese.
• For example, https://zh.wikipedia.org/wiki/%E4%BF%84%E7%BE%85%E6%96%AF%E9%A7%90%E8%8F%AF%E5%A4%A7%E4%BD%BF%E9%A4%A8

I consistently see Traditional Chinese characters (zh_Hant)—in this case, note that the fourth and fifth characters in the title are "駐華". Compare the Simplified Chinese (zh_Hans) page by adding the uselang=zh parameter, https://zh.wikipedia.org/wiki/%E4%BF%84%E7%BE%85%E6%96%AF%E9%A7%90%E8%8F%AF%E5%A4%A7%E4%BD%BF%E9%A4%A8?uselang=zh-CN . Two of the title characters are now "驻华".

This is a major issue for readability, and I suggest that we review the decision about which script should be the default. My suspicion is that we're relying on a software default rather than having made an intentional choice. I was personally surprised at the behavior, since ISO "zh" is Simplified Chinese, which has a much bigger reader base.