elementComputedLang.js

function computedLang(el){

  let root = el.closest('[lang]');
  let lang = root ? root.getAttribute('lang') : navigator.language;

  if (!el.matches(':lang('+lang+')')) {
    console.warn('lang missmatch!');
    for (let l of ['de','en','it','es', /*...*/ ]) if (el.matches(':lang('+l+')')) return l;
  }
  
  return lang;
}