code: def fix_missing_lang_tags(marc_xml, dom):
      
      def get_lang_tag(lang):
          lang_str = '\n  <mods:language>\n'
          lang_str += '    <mods:languageTerm authority="iso639-2b" type="code">'
          lang_str += lang
          lang_str += '</mods:languageTerm>\n'
          lang_str += '  </mods:language>\n\n'
  
          lang_dom = dhtmlparser.parseString(lang_str)
  
          return first(lang_dom.find("mods:language"))
  
      for lang in reversed(marc_xml["041a0 "]):
          lang_tag = dom.find(
              "mods:languageTerm",
              fn=lambda x: x.getContent().strip().lower() == lang.lower()
          )
          if not lang_tag:
              insert_tag(
                  get_lang_tag(lang),
                  dom.find("mods:language"),
                  get_mods_tag(dom)
              )
docs: If the lang tags are missing, add them to the MODS. Lang tags are parsed
      from `marc_xml`.