Symfony version(s) affected: >=4.1

Description
Symfony 4.1 introduced Internationalized routing.
After defining a route with multiple locale I tried to generate a url with a command and got the following error:

Unable to generate a URL for the named route "locale" as such route does not exist.

The route:

    /**
     * @Route({
     *    "en": "/locale",
     *    "hr": "/lokalno"
     * }, name="locale")
     */

The code that generates the url:

$this->router->generate('locale');

After some playing around I realized that I need to explicitly set the locale in order for the url to be generated, which was strange since I had the framework.default_locale set to hr.

// This works
$this->router->generate('locale.hr');

I did a little digging and noticed that the UrlGenerator class has a $defaultLocale parameter and the following code:

$locale = $parameters['_locale']
    ?? $this->context->getParameter('_locale')
    ?: $this->defaultLocale;

The problem is that the $defaultLocale parameter is never set when the class is instantiated. I would expect the framework.default_locale parameter to be passed here.

How to reproduce

# framework.yaml
framework:
    default_locale: hr

// Controller
class LocaleController extends AbstractController
{
    /**
     * @Route({
     *    "en": "/locale",
     *    "hr": "/lokalno"
     * }, name="locale")
     */
    public function index()
    {
        // ...
    }
}

// Command
class LocaleCommand extends Command
{
    protected static $defaultName = 'app:locale';

    // ...

    protected function execute(InputInterface $input, OutputInterface $output)
    {
        $output->writeln($this->router->generate('locale'));
    }
}

Possible Solution

A possible solution would be to override the way the UrlGenerator class is instantiated in the FrameworkBundle Router class and pass the framework.default_locale as the $defaultLocale parameter.