Unspecified nodes should be linked to the current URL language

Problem/Motivation

TODO: Write a more clear issue summary.

When you have enabled multilingual on a site, a lot of existing content is still marked as unspecified for its langcode, OR, these entity types are not marked as translatable.
Given that you goto a /fr page, and you have a link to another entity, which is not specified, ideally the URLs should contain /fr as well

Proposed resolution

Check for unspecified languages specificely, before making the decision about the URL language outbouding.

Remaining tasks

User interface changes

API changes

Data model changes