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var language = window.navigator.userLanguage || window.navigator.language;
$('#lang').append(language);
if ($.datepicker.regional[language] != undefined) $.datepicker.setDefaults($.datepicker.regional[language]);
else if ($.datepicker.regional[language.substring(0, 2)] != undefined) $.datepicker.setDefaults($.datepicker.regional[language.substring(0, 2).toLowerCase()]);
else $.datepicker.setDefaults($.datepicker.regional['']);
$('#myDate').datepicker({
    clickInput: true
});
<body>
    <p>Language is: <span id="lang"></span>

    </p>
    <div class="input-prepend"> <span class="add-on"><i class="icon-calendar"></i></span>

        <input class="span2"
        id="myDate" type="text"></input>
    </div>
</body>

              

              

              

                
External resources loaded into this fiddle: