Build the structured $content property on the entity.
array $entities: The entities, implementing EntityInterface, whose content is being built.
string $view_mode: (optional) The view mode that should be used to build the entity.
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(), $view_mode = 'full', $langcode = NULL);