How to handle a node translation containing an entity reference with a managed field property
There are a number of issues with reference counting files: see this overview. You might be affected by $entity->isDefaultTranslation() behaves incorrectly when changing default translation, causing file/image field usage to be set to zero, causing files to be deleted.

Note that due to the potential for lost data, as of 8.4 the default behaviour has been changed to not delete files even if their ref count reaches zero. See the CR for more.