Multisite Auto Language Switcher is a handy extensions for Multisite Language Switcher, which adds an automatic redirect to the preferred language of a user. To do so, it uses the preferred language the user has defined in its browser, which it automatically transmits on every request, to determine a matching language within your website. If a match could be found, it automatically redirects the user to this variant of the content.
Note: This plugin requires the plugin Multisite Language Switcher.