ese-website/_includes/header.njk

<header>
  <div class="header-container">
    <a href="{{ "/" | locale_url }}" title="home">ESE Logo</a>
    <nav id="mainNav">
      {{ collections.mainNav | pageLang | eleventyNavigation | eleventyNavigationToHtml | safe }}
    </nav>
  </div>
</header>

{% css %}
  {% include "header.css" %}
{% endcss %}