Symfony 4 use dashes in locales for subtags as per RFC5646

LocaleListener.php

<?php

namespace App\Listener;

use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken;

class LocaleListener {

    /**
     *
     * @var \Symfony\Component\DependencyInjection\ContainerInterface $container
     */
    protected $container;

    /**
     * Construct the listener
     * @param \Symfony\Component\DependencyInjection\ContainerInterface $container 
     */
    public function __construct(ContainerInterface $container) { // this is @service_container
        $this->container = $container;
    }

    /**
     * Listen for request events
     * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
     */
    public function onKernelRequest(GetResponseEvent $event) {
        if (!$event->isMasterRequest()) {
            // don't do anything if it's not the master request
            return;
        }
        $request = $event->getRequest();
        $request->setLocale(str_replace('_', '-', $request->getLocale()));
    }
}

services.yaml

services:
    app.locale_listener:
        class: App\Listener\LocaleListener
        arguments: ['@service_container']
        tags:
        - { name: kernel.event_listener, event: kernel.request, method: onKernelRequest }