DanCoulter

Once activated, this plugin will translate every instance of the jQuery UI 
Datepicker plugin on your blog. It even comes with the latest version of 
the jQuery plugin, so that you can add it to pages on your blog.
This will only work with version of the Datepicker plugin that come with 
jQuery UI versions 1.5 or higher.

Datepicker i18n

Globally translate your jQuery UI Datepicker instances.

Aaron Edwards

Tired of your custom plugin and theme translation files getting wiped out every time you use the WordPress auto-updater? Install and activate this plugin and 
WordPress will look for missing translation files in the WP_LANG_DIR where you keep your WordPress translation files.
<h4>UPDATE</h4>
In WordPress 3.8 and greater this plugin’s functionality is included in core. You can put translation files in: 
* Plugins: /wp-content/languages/plugins/textdomain-locale.mo 
* Themes: /wp-content/languages/themes/textdomain-locale.mo
A free plugin by Aaron Edwards of <a href="http://uglyrobot.com/" rel="nofollow ugc">UglyRobot Web Development</a>.

Translation Preserver

A simple plugin that helps preserve your custom plugin translations through auto-updates by allowing you to put them in your WP languages folder.

Datepicker i18n vs Translation Preserver