How to migrate multilingual content from CSV to Drupal 8?
you need both migration-tasks, the yml files in the migration folder:

migrate.migration.external_translated_test_node.yml
migrate.migration.external_translated_test_node_translation.yml


Then in the Plugin (MigrateExternalTranslatedTestSource.php), you need to read your .csv (in initializeIterator()) into the structure of the import array:

  /**
   * The data to import.
   *
   * @var array
   */
   protected $import = [
    ['name' =&gt; 'cat', 'title' =&gt; 'Cat', 'lang' =&gt; 'English'],
    ['name' =&gt; 'cat', 'title' =&gt; 'Chat', 'lang' =&gt; 'French'],
    ['name' =&gt; 'cat', 'title' =&gt; 'Gato', 'lang' =&gt; 'Spanish'],
    ['name' =&gt; 'dog', 'title' =&gt; 'Dog', 'lang' =&gt; 'English'],
    ['name' =&gt; 'dog', 'title' =&gt; 'Chien', 'lang' =&gt; 'French'],
    ['name' =&gt; 'monkey', 'title' =&gt; 'Monkey', 'lang' =&gt; 'English'],
   ];