ul > li { background-color : red }
li:lang(en-GB) { background-color : lime }

<ul>
  <li lang="en-GB">This list item should be green because its language is
        British English</li>
  <li lang="en-GB-wa">This list item should be green because its language
        is British English (Wales)</li>
</ul>
<ol>
  <li lang="en-US">This list item should NOT be green because its language
       is US English</li>
  <li lang="fr">This list item should NOT be green because its language is
       French</li>
</ol>