Function to add translation object to the migrated entity.

MigrateBase.php

<?php
public function addLanguagesToEntityTranslations($entity_type, $entity) {

  // Get the basic information about the entity.
  $entity_info = entity_get_info($entity_type);
  
  // Get the existing site languages.
  $languages = array_keys(language_list());
  // Add to the entity translation object. Set the original language to 
  // the entity's default language.
  $entity->translations = (object) array(
    'original' => $entity->language,
  );
  $data = array();
  foreach($languages as $language) {
    // For each language set the data array.
    $data[$language] = array(
      'entity_type' => $entity_type,
      'entity_id' => $entity->{$entity_info['entity keys']['id']},
      'language' => $language,
      'source' => $language == $entity->language ? '' : $entity->language,
      'uid' => '0',
      'status' => '1',
      'translate' => '0',
    );
  }
  // Add the data array to the entity translation object.
  $entity->translations->data = $data;
  
  // Update the entity.
  entity_save($entity_type, $entity);
}