https://symfony.com/doc/master/translation/locale.html

The custom listener must be called before LocaleListener, which initializes the locale based on the current request. To do so, set your listener priority to a higher value than LocaleListener priority (which you can obtain running the debug:event kernel.request command).

And then in the linked example:

https://symfony.com/doc/master/session/locale_sticky_session.html

    public static function getSubscribedEvents()
    {
        return array(
            // must be registered after the default Locale listener
            KernelEvents::REQUEST => array(array('onKernelRequest', 15)),
        );
    }

Documentation says (in bold) the custom listener must be registered before the framework listener, the inline doc in the example says after, which is exactly the opposite.