While working on #1966436: Default *content* entity languages are not set for entities created with the API we figured that Drupal 8 misses the 'language' entity key, as it got introduced for Drupal 7 in #1495648: Introduce entity language support.
Right now Drupal 8 hard-codes "langcode" as language key, but it should respect the defined 'language' key as we do for the other keys.