The JQuery documentation on the web seems to play hide&seek for where the
i18n files for the datepicker are see e.g.
Where are the i18n files of jQuery UI datepicker >= 1.11.0
As of this posting the link:
https://raw.githubusercontent.com/jquery/jquery-ui/master/ui/i18n/datepicker-de.js
seems to be valid. If i included it it didn't work by itself. I ended up
using http://ajax.googleapis.com/ajax/libs/jqueryui/1.11.1/i18n/jquery-ui-i18n.min.js which is not the same version as the 1.12 jquery I am using but it seems to work.
Please note I am using iso date format 2017-03-18 as yy-mm-dd here instead of the german locale style.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>jQuery UI Datepicker - Default functionality</title>
<link rel="stylesheet"
href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<link rel="stylesheet" href="/resources/demos/style.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.11.1/i18n/jquery-ui-i18n.min.js"></script>
<script>
$(function() {
// https://stackoverflow.com/questions/494958/how-do-i-localize-the-jquery-ui-datepicker
// https://stackoverflow.com/questions/13993619/how-to-set-locale-format-from-datepicker
$(".datepicker").datepicker($.datepicker.regional['de']);
$(".datepicker").datepicker('option','dateFormat','yy-mm-dd')
});
</script>
</head>