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.

Nikolay Bachiyski

Sometimes we want the front end of our blog to be translated for the users, but keep the backend admin panel in English. Admin in English does just that.

Admin in English

Admin in English lets you have your administration panel in English, even if the rest of your blog is translated into another language.

Datepicker i18n vs Admin in English