The Content translation module requires entity definitions to provide information about translation handlers. If this information is missing, it will provide its own defaults. The Entity API is covered in Chapter 10, The Entity API, but we will quickly discuss how the content translation module interacts with the Entity API.

Content entity definitions can provide a translation handler. If not provided, it will default to \Drupal\content_translation\ContentTranslationHandler. A node provides this definition and uses it to place the content translation information into the vertical tabs.

The content_translation_metadata key defines how to interact with translation metadata information, such as flagging other entities as outdated. The content_translation_deletion key provides a form class to handle entity translation deletion.

Currently, as of 8.0.1, no core modules provide implementations that override the default content_translation_metadata or content_translation_deletion.