Language-prefixed alias paths cannot be correctly routed inside a request

This doesn't seem to fix the following use case:

• Add the Dutch language (default, no prefix) and English language (with a /en prefix).
• Edit a Dutch node, linking to a different Dutch page using the English admin language.
• When a module calls PathValidator::getUrlIfValidWithoutAccessCheck() for the link, you run into the same issue.

Found this while looking at 🐛 processUrl Cannot Find the Correct Entity in Multilingual Site Active .
We could use the language with an empty language prefix as the default langcode. I think that should fix the issue.

Attached is a patch that seems to solve the issue.

The Needs Review Queue Bot → tested this issue.

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Patch enhanced to coner also the getUrlIfValidWithoutAccessCheck()

#9: I think we should inject the language manager instead of calling \Drupal::languageManager() if this change is necessary. And continue from the previous merge request too.