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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>languages — match</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    function matchLocale(supported, fallback) {
      var prefs = navigator.languages.map(function (t) {
        return t.toLowerCase();
      });
      for (var i = 0; i < prefs.length; i++) {
        var tag = prefs[i];
        if (supported.indexOf(tag) !== -1) return tag;
        var primary = tag.split("-")[0];
        if (supported.indexOf(primary) !== -1) return primary;
      }
      return fallback;
    }
    document.getElementById("out").textContent =
      "prefs: " + JSON.stringify(navigator.languages) +
      "\npicked: " + matchLocale(["en", "fr", "es", "ja"], "en");
  </script>
</body>
</html>

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languages — match supported locales (Try It)