IE10 under Windows 8 produces zh-Hans-CN instead of zh-CN, zh-Hant-TW instead of zh-TW. It will make all programs that rely on HTTP_ACCEPT_LANGUAGE header to determine the wrong locale. For instance, the message will be in traditional Chinese instead of simplified Chinese if zh-Hans-CN is used.
Workaround is to use Library.setProperty("org.zkoss.web.preferred.locale", "zh_CN"); if that happened.