Make language_fallback_fix compatible with Drupal 11

I think nothing more than this should be necessary. However, the old DrupalCI unfortunately doesn’t support testing against Drupal 11, so this will be hard to tell without GitLab CI. But I think we can just commit this and then maybe catch any problems via Search API’s GitLab CI pipelines.

Since no-one objected I’m now just committing this.

Automatically closed - issue fixed for 2 weeks with no activity.