HTML Code

<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

  <meta name=viewport content="width=device-width,initial-scale=1">

  <title>lang — JavaScript</title>

  <style>body{font-family:system-ui,sans-serif;margin:0;padding:1.5rem;background:#f8fafc;color:#334155;line-height:1.7}h1{margin:0 0 1rem;font-size:1.25rem;color:#0f172a}.panel{max-width:520px;padding:1rem 1.25rem;background:#fff;border:1px solid #e2e8f0;border-radius:8px;font-size:.8125rem}button{margin-top:1rem;padding:.5rem 1rem;font:inherit;cursor:pointer;border-radius:6px;border:1px solid #cbd5e1;background:#fff}.hint{margin-top:1rem;font-size:.8125rem;color:#64748b;max-width:520px}</style>

</head>

<body>

  <h1>document.documentElement.lang</h1>

  <div class=panel>

    <p style=margin:0>Current lang: <strong id=show>en</strong></p>

  </div>

  <button type=button id=toggle>Switch to Spanish (es)</button>

  <script>var btn=document.getElementById("toggle"),show=document.getElementById("show");btn.addEventListener("click",(function(){var t="en"===document.documentElement.lang?"es":"en";document.documentElement.lang=t,show.textContent=t,btn.textContent="en"===t?"Switch to Spanish (es)":"Switch to English (en)"}))</script>

  <p class=hint>Updating <code>lang</code> at runtime helps when users change site language without a full reload.</p>

</body>

</html>

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HTML lang Attribute — Switch lang with JavaScript