title: Fullcalendar locale not changing header bar in v5
body: <p>I'm having issues changing the locale of the headerToolbar of the fullcalendar.</p>
  <p>What's weird is that it works on month and day names, just not the header.</p>
  <p><a href="https://i.stack.imgur.com/zEVgm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zEVgm.png" alt="enter image description here" /></a></p>
  <p>Code looks like this:</p>
  <pre><code>let calendar = new FullCalendar.Calendar(calendarEl, {
      headerToolbar: {
          locale: getLanguage(),
          left: 'prev,next',
          center: 'title',
          right: 'timeGridWeek,dayGridMonth,listWeek',
      },
      initialView: 'timeGridWeek',
      weekNumbers: true,
      weekText: &quot;&quot;,
      weekends: false,
      firstDay: 1,
      slotMinTime: &quot;07:00:00&quot;,
      slotMaxTime: &quot;18:00:00&quot;,
      eventTimeFormat: {
          hour: '2-digit',
          minute: '2-digit',
          hour12: false, 
      },
      locale: getLanguage(),
      eventDisplay: &quot;block&quot;,
      eventTextColor: &quot;#fff&quot;
  });
  </code></pre>
  <p>getLanguage() returns the browsers language, in my case it should (and do) return no-NB.</p>
  <pre><code>function getLanguage() {
      let result = window.localStorage[LANGUAGE];
      return result ? result : navigator.language || navigator.userLanguage || 'no';
  }
  </code></pre>
  <p>How come it doesn't change the localization of the headerToolbar?</p>