custom entity

   $content_entity_types = [];
   $entity_type_definations = \Drupal::entityTypeManager()->getDefinitions();
   /* @var $definition EntityTypeInterface */
   foreach ($entity_type_definations as $definition) {
     if ($definition instanceof ContentEntityType) {
       $content_entity_types[] = $definition;
     }
   }

/**
* Implements hook_theme().
*/
function mymodule_theme() {
 return [
   'my_entity' => [
     'render element' => 'elements',
   ],
 ];
}
 * Implements hook_preprocess_HOOK().
*/

Add to file src/Entity/MyEntity.php

/**
* Defines the my entity class.
*
* @ContentEntityType(
...
*   data_table = "my_entity_field_data",
*   translatable = TRUE,
...
*   entity_keys = {
...
*     "langcode" = "langcode"
*   },
...
* )
*/

Add ->setTranslatable(TRUE) to baseFieldDefinitions. For example