Datepicker jQuery UI ... set language

I tried to set Datepicker, like this:

<script type="text/javascript">
    $.datepicker.setDefaults($.datepicker.regional['nl']);
    $(function () {
        $("#tbDateDiagnostic").datepicker({
            numberOfMonths: 2,
            showButtonPanel: true,
            ateFormat: 'dd/mm/yy'
        });

        $("#tbDateSend").datepicker({
            numberOfMonths: 2,
            showButtonPanel: true,
            dateFormat : 'dd/mm/yy'
        });
    });
</script>

But all the time, the datepicker is in French and not in Dutch.

Did I do something wrong?