Import translations into Drupal from a .po file

import-translations.php

<?php

//
// This code uses the _locale_import_po() function call from includes/locale.inc. This
// is a private function so use with care. Here's the function:
//   https://api.drupal.org/api/drupal/includes%21locale.inc/function/_locale_import_po/7
//

// example usage:
$translation_filepath = drupal_get_path('module', '<module>') . '/translations-es.po';
_import_translation($translation_filepath, 'es');

function _import_translation($path, $lang) {
  if (file_exists($path)) {
    // create a temp file in public:// with the translations
    $data = file_get_contents($path);
    $filename = "public://translations-$lang.po";
    $file = file_save_data($data, $filename, FILE_EXISTS_REPLACE);
    if ($file) {
     // import into Drupal
     _locale_import_po($file, $lang, LOCALE_IMPORT_OVERWRITE, 'default');
      // remove temp file
      file_delete($file);
    }   
  }
}