The name of the month is handled by your local PHP configuration. However you can try to override it by adding the following code to the top of /components/com_imagecalendar/imagecalendar.php after line 12, defined('_JEXEC') or die('Restricted access'); so it looks like this:
defined('_JEXEC') or die('Restricted access');
@setlocale(LC_ALL, 'ru_RU.ISO_8859-5');
Jared