Issue #2431329 by plach: Make (content) translation language available as a field definition

core/lib/Drupal/Core/Entity/ContentEntityBase.php

core/lib/Drupal/Core/Entity/EntityManager.php

core/lib/Drupal/Core/Entity/EntityType.php

core/lib/Drupal/Core/Entity/FieldableEntityInterface.php

core/lib/Drupal/Core/Entity/Query/Sql/Tables.php

core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php

.../Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php

core/lib/Drupal/Core/TypedData/TranslatableInterface.php

core/modules/block_content/src/Entity/BlockContent.php

core/modules/comment/src/Entity/Comment.php

...modules/content_translation/content_translation.admin.inc

.../modules/menu_link_content/src/Entity/MenuLinkContent.php

core/modules/node/src/Entity/Node.php

...dules/serialization/src/Tests/EntitySerializationTest.php

core/modules/shortcut/src/Entity/Shortcut.php

...modules/system/src/Tests/Entity/EntityTranslationTest.php

...ystem/tests/modules/entity_test/src/Entity/EntityTest.php

...dules/entity_test/src/Entity/EntityTestMulLangcodeKey.php

core/modules/taxonomy/src/Entity/Term.php

core/modules/user/src/AccountForm.php