drupal/core/modules/locale/locale.bulk.js

/**
* DO NOT EDIT THIS FILE.
* See the following change record for more information,
* https://www.drupal.org/node/2815083
* @preserve
**/

(function ($, Drupal) {
  Drupal.behaviors.importLanguageCodeSelector = {
    attach: function attach(context, settings) {
      var $form = $('#locale-translate-import-form').once('autodetect-lang');
      if ($form.length) {
        var $langcode = $form.find('.langcode-input');
        $form.find('.file-import-input').on('change', function () {
          var matches = $(this).val().match(/([^.][\.]*)([\w-]+)\.po$/);
          if (matches && $langcode.find('option[value="' + matches[2] + '"]').length) {
            $langcode.val(matches[2]);
          }
        });
      }
    }
  };
})(jQuery, Drupal);