Variables added with form alter are not shown as translatable, needs better i18n_variable_form_alter()

I have problem with translation of variable.

1. First I add some form

<?php
/*
* Implementation of hook_form_alter
*/
  
  function some_modules_form_alter(&$form, &$form_state, $form_id) {
       if('system_site_information_settings' == $form_id) {

          $form['site_information']['site_footer'] = array(
            '#type' => 'textfield',
            '#title' => t('Footer text'),
            '#default_value' => variable_get('site_footer', 'Default text'),
            '#description' => t('Some text'),
            '#required' => TRUE
            );
            
        }

        
        return system_settings_form($form);

  }
?>

2. Then I create variable definition

<?php

 /**
  * Implementation of hook_variable_info
  */ 
  
 function  some_modules_variable_info($options) {

     
 $variables['site_footer'] = array(
    'type' => 'string',
    'title' => t('Footer text', array(), $options),
    'default' => 'Default text',
    'description' => t('Some text', array(), $options),
    'required' => TRUE,
    'group' => 'site_information',
    );

     return $variables;
 }
?>

3. I saw 'Footer text' variable in Multilingual variable settings. I selected checkbox and I push submit form.

4. I returned to "Site information page", but still I don't have in description "This is a multilingual variable.", and variable is still the same for all language.

What I forgot to do?

Thanx for help,