Issue #3111571: Except for websites created by Drupal, no websites use "zh-hans" and "zh-hant" language codes.
Browsers understand language codes differently to Drupal. IE and Firefox will send 'zh-cn', but Drupal understands this language as 'zh-hans'. for Simplified Chinese langugage, both IE and Firefox will send 'zh-cn' as the preferred language code, which is different from Drupal's 'zh-hans' code, thus the browser detection will not work for Simplified Chinese.The official website of the United Nations also uses "zh" instead of "zh-hans",zh uses Chinese Chinese (zh-cn) by default,and using zh-cn is more common in the browser.