NotificationType.php

$form->add(
            $bundle_key,
            'choice',
            array(
                'label' => 'Bundle languages ' .$bundle_key,
                'choices' => array(
                    '' => 'not defined',
                    'it' => 'it',
                    'en' => 'en',
                ),
                'multiple' => true,
                'expanded' => true,
            )
        );