We also offer support for free plugins at http://wordpress.dwbooster.com/contact-us
The current version (as today March 30, 2013) of the plugin uses the YUI (Yahoo! User Interface) to render the calendar, but note that it isn't connected or associated to the Yahoo! Widget calendar in any other way. In order to make it load faster, the next version of the plugin will render the calendar using only with jQuery (already loaded in most WP websites).
If you want to add a new language you can edit the file "TDE_RCalendar/all-scripts.js", about line #171, into the function "configLanguage". You will see the current languages into that function, you can either replace one of the existing languages there or add a new one. Probably the easiest method is to replace one of the existing languages.
Once again, the above solution is temporal since we plan to publish within a few weeks the new version using jQuery to render the calendar that will include a different method for editing languages.
Thank you for using the plugin!