Problem/Motivation
Following #2467827: [META] W3C validation for Drupal Core, the W3C validator triggers an error for links generated for route . Unfortunately, the LinkGenerator.hp method adds the hreflang attributes in all cases while are made with tags which does not support the hreflang attribute.
The solution is to unset the hreflang in the same way it is done with href in the case of route.