README.md

drush --yes translation-import --language=de --customized=true sites/default/translations/de.po

https://github.com/dawehner/drush_language

translation.drush.inc

<?php

/**
 * @file
 * Custom drush commands.
 */

/**
 * Implements hook_drush_command().
 *
 * @return array
 */
function translation_drush_command() {
  $items['translation-import'] = array(
    'description' => 'Import custom translations',
    'arguments' => array(
      'file' => 'The po file.',
    ),
    'options' => array(
      'language' => array(
        'required' => TRUE,
      ),
      'customized' => array(
        'required' => TRUE,
      ),
    ),
    'required-arguments' => TRUE,
  );
  return $items;
}

/**
 * Import translations.
 */
function drush_translation_import($filepath) {
  if (!file_exists($filepath)) {
    drush_set_error('error', '*.po file not found.');
    return;
  }

  $file = new stdClass();
  $file->langcode = drush_get_option('language');
  $file->uri = $filepath;

  Drupal\locale\Gettext::fileToDatabase($file, ['customized' => (int) drush_get_option('customized')]);
}