The issue is in the following function:

field_collection_i18n_string_list_field_alter(...)

in the block

      foreach ($instance['display'] as $view_mode => $display) {
        if ($display['type'] != 'field_collection_fields') {
  $display['settings'] += array('edit' => 'edit', 'delete' => 'delete', 'add' => 'add');
          $properties['field'][$instance['field_name']][$instance['bundle']]['setting_edit'] = array(
            'title' => t('Edit link title'),
            'string' => $display['settings']['edit'],
          );
          $properties['field'][$instance['field_name']][$instance['bundle']]['setting_delete'] = array(
            'title' => t('Delete link title'),
            'string' => $display['settings']['delete'],
          );
          $properties['field'][$instance['field_name']][$instance['bundle']]['setting_add'] = array(
            'title' => t('Add link title'),
            'string' => $display['settings']['add'],
          );
        }
      }

the preexistence of $display['settings'] is not verified prior to merging the array.

I propose to add a check for the existence of the array. settings should be an array, but in case it is not, we wipe it clean and provide it.

/**
* Implements hook_i18n_string_list_{textgroup}_alter().
*/
function field_collection_i18n_string_list_field_alter(&$properties, $type, $instance) {
  if ($type == 'field_instance') {
    $field = field_info_field($instance['field_name']);
    if ($field['type'] == 'field_collection' && !empty($instance['display'])) {
      foreach ($instance['display'] as $view_mode => $display) {
        if ($display['type'] != 'field_collection_fields') {
          if (! isset($display['settings']) || ! is_array($display['settings'])) {
            $display['settings'] = array();
          }
          $display['settings'] += array('edit' => 'edit', 'delete' => 'delete', 'add' => 'add');
          $properties['field'][$instance['field_name']][$instance['bundle']]['setting_edit'] = array(
            'title' => t('Edit link title'),
            'string' => $display['settings']['edit'],
          );
          $properties['field'][$instance['field_name']][$instance['bundle']]['setting_delete'] = array(
            'title' => t('Delete link title'),
            'string' => $display['settings']['delete'],
          );
          $properties['field'][$instance['field_name']][$instance['bundle']]['setting_add'] = array(
            'title' => t('Add link title'),
            'string' => $display['settings']['add'],
          );
        }
      }
    }
  }
}