<!DOCTYPE html>
<html>

  {% include head.html %}

  <body>

    {% include header.html %}

    <div class="page-content">
      <div class="wrapper">
        {{ content }}
      </div>
    </div>

    {% include footer.html %}

  </body>

</html>

<!DOCTYPE html>
<html lang="en">

  {% include head.html %}

  <body>

    {% include header.html %}

    <div class="page-content">
      <div class="wrapper">
        {{ content }}
      </div>
    </div>

    {% include footer.html %}

  </body>

</html>

Add the lang attribute to <html>
Add the lang attribute to <html>

More on this:

https://developer.paciellogroup.com/blog/2016/06/using-the-html-lang-attribute/