Notice: Undefined index: "Add a role to the selected users"

If using a language settig not equal to 'en' this notice will occur in line 155 and further.
The grouping keys 'Add a role to the selected users' and 'Remove a role from the selected users' should be translated e.g like this:

/**
 * Alters user_admin_account
 */
function user_permissions_form_user_admin_account_alter(&$form, &$form_state, $form_id) {
  $groupadd = t('Add a role to the selected users');
  $groupremove  = t('Remove a role from the selected users');
  if (is_array($form['options']['operation']['#options'][$groupadd]) &&
      is_array($form['options']['operation']['#options'][$groupremove])) {

    // Removes _user_role_N roles from the 'Add a role to the selected users' Update Option on the /admin/people page
    foreach ($form['options']['operation']['#options'][$groupadd] as $key => $value) {
      if (preg_match('/^_user_role_\d+$/', $value)) {
        unset($form['options']['operation']['#options'][$groupadd][$key]);
      }
    }
    
    // Removes _user_role_N roles from the 'Remove a role from the selected users' Update Option on the /admin/people page
    foreach ($form['options']['operation']['#options'][$groupremove] as $key => $value) {
      if (preg_match('/^_user_role_\d+$/', $value)) {
        unset($form['options']['operation']['#options'][$groupremove][$key]);
      }
    }
  }
}

may be a better solution is to use count($roles) instead of is_array(), see users.module line 3099