How do I localize the jQuery UI Datepicker?

For those that still have problems, you have to download the language file your want from here:

https://github.com/jquery/jquery-ui/tree/master/ui/i18n

and then include it in your page like this for example(italian language):

<script type="text/javascript" src="/scripts/jquery.ui.datepicker-it.js"></script>

then use zilverdistel's code :D

I figured out the demo and implemented it the following way:

$.datepicker.setDefaults(
  $.extend(
    {'dateFormat':'dd-mm-yy'},
    $.datepicker.regional['nl']
  )
);

I needed to set the default for the dateformat too ...

The string $.datepicker.regional['it'] not translate all words.

For translate the datepicker you must specify some variables:

$.datepicker.regional['it'] = {
    closeText: 'Chiudi', // set a close button text
    currentText: 'Oggi', // set today text
    monthNames: ['Gennaio','Febbraio','Marzo','Aprile','Maggio','Giugno',   'Luglio','Agosto','Settembre','Ottobre','Novembre','Dicembre'], // set month names
    monthNamesShort: ['Gen','Feb','Mar','Apr','Mag','Giu','Lug','Ago','Set','Ott','Nov','Dic'], // set short month names
    dayNames: ['Domenica','Luned&#236','Marted&#236','Mercoled&#236','Gioved&#236','Venerd&#236','Sabato'], // set days names
    dayNamesShort: ['Dom','Lun','Mar','Mer','Gio','Ven','Sab'], // set short day names
    dayNamesMin: ['Do','Lu','Ma','Me','Gio','Ve','Sa'], // set more short days names
    dateFormat: 'dd/mm/yy' // set format date
};

$.datepicker.setDefaults($.datepicker.regional['it']);

$(".datepicker").datepicker();

In this case your datepicker is properly translated.

  $.datepicker.setDefaults({
    closeText: "关闭",
    prevText: "<上月",
    nextText: "下月>",
    currentText: "今天",
    monthNames: [ "一月","二月","三月","四月","五月","六月",
    "七月","八月","九月","十月","十一月","十二月" ],
    monthNamesShort: [ "一月","二月","三月","四月","五月","六月",
    "七月","八月","九月","十月","十一月","十二月" ],
    dayNames: [ "星期日","星期一","星期二","星期三","星期四","星期五","星期六" ],
    dayNamesShort: [ "周日","周一","周二","周三","周四","周五","周六" ],
    dayNamesMin: [ "日","一","二","三","四","五","六" ],
    weekHeader: "周",
    dateFormat: "yy-mm-dd",
    firstDay: 1,
    isRTL: false,
    showMonthAfterYear: true,
    yearSuffix: "年"
  });

the i18n code could be copied from https://github.com/jquery/jquery-ui/tree/master/ui/i18n

1. You need to load the jQuery UI i18n files:

<script src="//ajax.googleapis.com/ajax/libs/jqueryui/1.11.4/i18n/jquery-ui-i18n.min.js">
</script>

2. Use $.datepicker.setDefaults function to set defaults for ALL datepickers.

3. In case you want to override setting(s) before setting defaults you can use this:

var options = $.extend(
    {},                                  // empty object
    $.datepicker.regional["fr"],         // fr regional
    { dateFormat: "d MM, y" /*, ... */ } // your custom options
);
$.datepicker.setDefaults(options);

The order of parameters is important because of the way jQuery.extend works. Two incorrect examples:

/*
 * This overwrites the global variable itself instead of creating a
 * customized copy of french regional settings
 */
$.extend($.datepicker.regional["fr"], { dateFormat: "d MM, y"});

/*
 * The desired dateFormat is overwritten by french regional 
 * settings' date format
 */
$.extend({ dateFormat: "d MM, y"}, $.datepicker.regional["fr"]);

Here is example how you can do localization without some extra library.

jQuery(function($) {
  $('input.datetimepicker').datepicker({
    duration: '',
    changeMonth: false,
    changeYear: false,
    yearRange: '2010:2020',
    showTime: false,
    time24h: true
  });

  $.datepicker.regional['cs'] = {
    closeText: 'Zavřít',
    prevText: '<Dříve',
    nextText: 'Později>',
    currentText: 'Nyní',
    monthNames: ['leden', 'únor', 'březen', 'duben', 'květen', 'červen', 'červenec', 'srpen',
      'září', 'říjen', 'listopad', 'prosinec'
    ],
    monthNamesShort: ['led', 'úno', 'bře', 'dub', 'kvě', 'čer', 'čvc', 'srp', 'zář', 'říj', 'lis', 'pro'],
    dayNames: ['neděle', 'pondělí', 'úterý', 'středa', 'čtvrtek', 'pátek', 'sobota'],
    dayNamesShort: ['ne', 'po', 'út', 'st', 'čt', 'pá', 'so'],
    dayNamesMin: ['ne', 'po', 'út', 'st', 'čt', 'pá', 'so'],
    weekHeader: 'Týd',
    dateFormat: 'dd/mm/yy',
    firstDay: 1,
    isRTL: false,
    showMonthAfterYear: false,
    yearSuffix: ''
  };

  $.datepicker.setDefaults($.datepicker.regional['cs']);
});

<!DOCTYPE html>
<html>

<head>
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
  <link data-require="jqueryui@*" data-semver="1.10.0" rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/jqueryui/1.10.0/css/smoothness/jquery-ui-1.10.0.custom.min.css" />
  <script data-require="jqueryui@*" data-semver="1.10.0" src="//cdnjs.cloudflare.com/ajax/libs/jqueryui/1.10.0/jquery-ui.js"></script>
  <script src="datepicker-cs.js"></script>
  <script type="text/javascript">
    $(document).ready(function() {
      console.log("test");
      $("#test").datepicker({
        dateFormat: "dd.m.yy",
        minDate: 0,
        showOtherMonths: true,
        firstDay: 1
      });
    });
  </script>
</head>

<body>
  <h1>Here is your datepicker</h1>
  <input id="test" type="text" />
</body>
</html>

If you use jQuery UI's datepicker and moment.js on the same project, you should piggyback off of moment.js's locale data:

// Finnish. you need to include separate locale file for each locale: https://github.com/moment/moment/tree/develop/locale
moment.locale('fi'); 

// fetch locale data internal structure, so we can shove it inside jQuery UI
var momentLocaleData = moment.localeData(); 

$.datepicker.regional['user'] = {
    monthNames: momentLocaleData._months,
    monthNamesShort: momentLocaleData._monthsShort,
    dayNames: momentLocaleData._weekdays,
    dayNamesShort: momentLocaleData._weekdaysMin,
    dayNamesMin: momentLocaleData._weekdaysMin,
    firstDay: momentLocaleData._week.dow,
    dateFormat: 'yy-mm-dd' // "2016-11-22". date formatting tokens are not easily interchangeable between momentjs and jQuery UI (https://github.com/moment/moment/issues/890)
};

$.datepicker.setDefaults($.datepicker.regional['user']);

just in case anyone is STILL stuck on this, despite the other answers, I solved this with:

$.datepicker.setDefaults($.datepicker.regional['en-GB']);

note the extra 'GB'. After that it worked fine.

This solution may help.

$(document).ready(function () {
  var userLang = navigator.language || navigator.userLanguage;

  var options = $.extend({},
    $.datepicker.regional["ja"], {
      dateFormat: "yy/mm/dd",
      changeMonth: true,
      changeYear: true,
      highlightWeek: true
    }
  );

  $("#japaneseCalendar").datepicker(options);
});

#ui-datepicker-div {
  font-size: 14px;
}

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <link rel="stylesheet" type="text/css"
          href="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.12.1/themes/smoothness/jquery-ui.min.css">
    <script src="https://code.jquery.com/jquery-3.2.1.min.js"></script>
    <script src="https://code.jquery.com/ui/1.12.1/jquery-ui.min.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.11.1/i18n/jquery-ui-i18n.min.js"></script>
</head>
<body>
<h3>Japanese JQuery UI Datepicker</h3>
<input type="text" id="japaneseCalendar"/>

</body>
</html>

<link rel="stylesheet" href="//code.jquery.com/ui/1.11.4/themes/smoothness/jquery-ui.css">

<script src="//code.jquery.com/ui/1.11.4/jquery-ui.js"></script>
<script src=">/js/datepicker-fr.js"></script>
<script>
jQuery(function() {
jQuery( "#datepicker" ).datepicker({minDate: 0 , dateFormat: 'mm/dd/yy'});
});

</script>

<script type="text/javascript">
$(document).ready(function(){
$('#datepicker').datepicker($.datepicker.regional['fr']);
});
</script>