static getLanguagesFrom_(tracks) {
    const languages = new Set();

    for (const track of tracks) {
      if (track.language) {
        languages.add(shaka.util.LanguageUtils.normalize(track.language));
      } else {
        languages.add('und');
      }
    }

    return languages;
  }
Get the normalized languages for a group of tracks.

@param {!Array<?(shaka.extern.Track|shaka.extern.TextTrack)>} tracks
@return {!Set<string>}
@private