code: function localeSupported(locale) {
    const language = locale.split('-')[0];
    if (language === 'en') {
      // Assume all flavors of English are supported.
      return true;
    }
    // Try formatting a Tuesday date, and if we get the English result "Tue",
    // the browser probably doesn't support the locale, and used the default
    // locale instead.
    const date = new Date('10 March 2015'); // A Tuesday
    const formatter = new Intl.DateTimeFormat(locale, { weekday: 'short' })
    const result = formatter.format(date);
    return result !== 'Tue';
  }
docs: Heuristic that returns true if the given locale is supported.