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const defaultLanguage = () => {
  let language = window.navigator.userLanguage 
              || window.navigator.language 
              || window.navigator.browserLanguage
              || window.navigator.systemLanguage
              || 'en';
  const regexp = /([a-z]{2}-)([a-z]{2})/;
  if (regexp.test(language)) {
    language = language.replace(regexp, function(match, ...parts) {
      return parts[0] + parts[1].toUpperCase();
    });
  }
  return language;
};