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

The custom listener must be calledbefore 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

publicstaticfunctiongetSubscribedEvents()    {returnarray(// must be registered after the default Locale listener            KernelEvents::REQUEST =>array(array('onKernelRequest',15)),        );    }

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