Русификация интерфейса

Названия месяцев как и сам виджет полностью локализованы в официальной русской сборке Wrodpress

I18n for widgets developed on 2.8+

WordPress 2.8+ uses a new Widget API, that only requires the widget developer to extend the standard widget class and some of its functions. With this API there is no init function. After the widget is coded using the widget(), form(), and update() methods, the widget must be registered. The textdomain is then loaded after the widget is registered.

Example:

// register Foo_Widget widget
function Foo_Widget_init() {
    return register_widget( 'Foo_Widget' );
}
add_action( 'widgets_init', 'Foo_Widget_init' );

$plugin_dir = basename( dirname( __FILE__ ) );
load_plugin_textdomain( 'foo_widget', null, $plugin_dir );

This example registers a widget named Foo_Widget, then sets the plugin directory variable and attempts to load the foo_widget-locale.po file.