Symfony version(s) affected: 4.1.5

Description
When defining i18n routes, with only a language and not a combination of language and region, the fallback mechanism via _canonical_route does not work, when the locale contains both language and region.

How to reproduce

locale: en_US

petition_show:
    path:
        de: /petitionen/{id}/{slug}
        en: /petitions/{id}/{slug}

$this->generateUrl('petition_show', ...

Gives the error:
Unable to generate a URL for the named route "petition_show" as such route does not exist.

Possible Solution

UrlGenerator::generate should check both the full locale and only the language part of the locale.
This would have to be reflected in PhpGeneratorDumper::generateGenerateMethod.

Additional context

This would be the same behaviour as the for example the translator works.