Relocate langcode where it used to be entity.langcode

Marc Orcaurequested to merge
3111456-language-issue-5.0.6-relocate-langcode-in-entity into 3111456-language-issue-5.0.6

Relocates langcode key where it used to be as per comment https://www.drupal.org/project/decoupled_router/issues/3111456#comment-16317492

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