D8_rplf/core/modules/locale/locale.bulk.js

/**
 * @file
 * Locale behavior.
 */

(function ($, Drupal) {

  'use strict';

  /**
   * Select the language code of an imported file based on its filename.
   *
   * This only works if the file name ends with "LANGCODE.po".
   *
   * @type {Drupal~behavior}
   *
   * @prop {Drupal~behaviorAttach} attach
   *   Attaches behavior for preselecting language code based on filename.
   */
  Drupal.behaviors.importLanguageCodeSelector = {
    attach: function (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 () {
            // If the filename is fully the language code or the filename
            // ends with a language code, pre-select that one.
            var matches = $(this).val().match(/([^.][\.]*)([\w-]+)\.po$/);
            if (matches && $langcode.find('option[value="' + matches[2] + '"]').length) {
              $langcode.val(matches[2]);
            }
          });
      }
    }
  };

})(jQuery, Drupal);