Build the structured $content property on the entity.
array $entities: The entities, implementing EntityInterface, whose content is being built.
array $displays: The array of entity_display objects holding the display options configured for the entity components, keyed by bundle name.
string $view_mode: The view mode in which the entity is being viewed.
string $langcode: (optional) For which language the entity should be build, defaults to the current content language.
array The content array.
public function buildContent(array $entities, array $displays, $view_mode, $langcode = NULL);