auto-captions.js

function getParameterByName(name) {
  name = name.replace(/[\[]/, "\\[").replace(/[\]]/, "\\]");
  var regex = new RegExp("[\\?&]" + name + "=([^&#]*)"),
      results = regex.exec(location.search);
  return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
}

window.videojs.plugin('autoCaptions', function () {
  var player = this,
      langMap = {
        'de-de': 'German',
        'es-es': 'Spanish',
        'fr-fr': 'French',
        'ja-jp': 'Japanese',
        'ko-kr': 'Korean',
        'pt-br': 'Portuguese',
        'zh-cn': 'Chinese'
      },
      options = player.options(),
      dataCaptionsLang = options['data-captions-lang'],
      langParam = getParameterByName('_lang'),
      lang = langMap[dataCaptionsLang || langParam] || false,
      captions;

  player.on('play', function () {
    captions = player.textTracks();
    for (var i = 0; i < captions.length; i++) {
      if (lang === captions[i].language) {
        captions[i].mode = 'showing';
      }
    }
  });
});