What do you mean exactly by it fails ? What is happening ?

Stack trace: https://pastebin.com/hbvFgUiu

To be 100% clear: this template renders fine when it is rendered from a controller. The error just occurs when I do the rendering from the EventListener that reacts to 404.

Presumably some initialization has not happened yet at that point, or maybe I am accessing the wrong twig instance or something like that. As mentioned, I looked though the Symfony source a bunch though it's hard to follow things due to magical dependency injection which I am not used to.

I guess the problem is that the exception occurs before the LocaleListener is executed. This means that the URL generator is not able to find a route as there is no locale set yet.

Status: Reviewed

Exactly xabbuh. Just reproduced it successfully.

RouterListener is executed before LocaleListener so there is no locale in the RequestContext in the generator.

However, there is a little fix we can do. We currently don't pass a default locale to the generator. We should pass the kernel.default_locale param I guess as an ultimate fallback. But still, every generated link will be for the "en" locale for example.

If you rely on the locale being extracted from the route configuration, I think we cannot do better as there is no matching route that could be used to extract the locale. When using something more sophisticated with custom code (e.g. storing the locale in the session), you could of course make sure that your own listener is executed before.

I thought about my proposition a bit more and actually it would lead to BC breaks if there is another non internationalized route with the same name. Exemple : "myroute" and "myroute.en"

I got around the linking via path() issue by injecting the UrlGenerator into my EventListener and setting the locale parameter in its RequestContext.

$this->urlGenerator->getContext()->setParameter(
    '_locale',
    $locale
);

While that solves the linking issue, the template is rendered with the default locale, rather than the one I am computing from the url in my EventListener.

I've switched to using some catch-all routes for 404 rather than using an EventListener. This approach is much simpler and works nicely with the new Symfony internationalized routing (unlike the JMS bundle where I did not get it to work for the life of me). So this bug is no longer getting in my way. Still, it seems like the route function / UrlGenerator should at least fall back to the default locale and not error out.

Does #31023 help?

I ran composer update and am now rendering a 500 page from the EventListener without any problems, so I guess this is fixed.

Your problem might be having yet another locale set. I've encourted similar issue here:
https://stackoverflow.com/questions/63286922/symfony-4-multilang-routes-with-default-fallback

Trying to workaround this.