div.stub * { background-color : red  }
div.stub *:not(:lang(fr)) { background-color : green }

<div lang="en" class="stub">
<p>This paragraph should have a green background because the
   enclosing div is in english.</p>
<p lang="en">This paragraph should have a green background because
   it is in english.</p>
<div lang="fr">
  <p lang="en">This paragraph should have a green background
     because it is in english.</p>
</div>
</div>