HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>lang prefix demo</title>
  <style>
    body { font-family: sans-serif; padding: 16px; max-width: 28rem; }
    div { padding: 8px; margin: 6px 0; border: 1px solid #e2e8f0; border-radius: 6px; }
    #output { margin-top: 12px; padding: 12px; background: #fef3c7; font-family: monospace; white-space: pre-wrap; }
  </style>
</head>
<body>
  <div lang="en">Generic English</div>
  <div lang="en-US">US English</div>
  <div lang="fr">French</div>
  <div id="output"></div>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("#output").text(
      ":lang(en) → " + $("div:lang(en)").length + "\n:lang(en-us) → " + $("div:lang(en-us)").length
    );
  </script>
</body>
</html>

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:lang(en) vs :lang(en-us) (Try It)