Cannot change homepage language

hi, my site is in two languages, English (default) & Chinese, i have enabled the browser detection to detect the language setting, when chinese setting is detected, the site is redirected to http://xxx.com/zh, but when that user clicks on “English” language switch button on main menu, it still redirects to the Chinese site (http://xxx.com/zh) instead of http://xxx.com/

please kindly advise how to fix it, thanks.