WordPress po files

The define('WPLANG', 'ru_RU'); in the wp-config.php is no longer needed, as WordPress stores this value in the database (starting from version 4.0).

However, I suppose your problem is actually something different.

Your Steps:

Add a filter to locale

To ensure your language is set correctly, use a code like this:

add_filter( 'locale', 'f711_set_language' );
function f711_set_language( $locale ) {

    // you can use any switches to define the language here
    return 'ru_RU';

}

Use a textdomain

Change your Code to ensure your localized output is in a textdomain

<?php _e( 'zibil', 'your_textdomain' ); ?>

Be sure that you load the textdomain correctly

For Themes, you can use this. Be sure to define the right directory where your translation files are stored.

add_action('after_setup_theme', 'f711_load_theme_textdomain');
function f711_load_theme_textdomain(){
    load_theme_textdomain( 'your_textdomain', get_template_directory().'/languages' );
}

For Plugins, use this:

add_action( 'plugins_loaded', 'f711_load_plugin_textdomain' );
function f711_load_plugin_textdomain() {
    load_plugin_textdomain( 'your_textdomain', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' ); 
}

Be sure you named your .po correctly

The translation file HAS to be called ru_RU.po in your case.

You should be set up correctly afterwards.