Loading custom language file for plugin

If you will use the translation only in back end, then load the translation file on the hook admin_init. If not, also on front end, then use the hook init.

A example:

class Fb_Example_Class {

    /**
     * Constructor, init the functions inside WP
     *
     * @return  \Fb_Example_Class 
     */
    private function __construct() {

        // load translation files
        add_action( 'admin_init', array( $this, 'localize_plugin' ) );
    }

    /**
     * Localize_plugin function.
     *
     * @uses    load_plugin_textdomain, plugin_basename
     * @access  public
     * @return  void
     */
    public function localize_plugin() {

        load_plugin_textdomain( 'textdomain', FALSE, dirname( plugin_basename( __FILE__ ) ) . '/languages' );
    }

}

You find also helpful content and examples on the codex page.