function getFileName(lang, i, isBeta, chapter) {
  if (i === 0 && lang === "en") {
    return "index.html";
  }
  let fileName = `${pad(i)}_${lang}.html`;
  if (chapter !== undefined && chapter !== null) {
    fileName = `chapter_${chapter}_${lang}.html`;
  }
  return fileName;
}
@param {string} lang
@param {number} i
@param {boolean} isBeta currently unused
@param {string} chapter
@returns {string}