add langua field to custom entity.

I have an entity call "armys", and I want to add a field to be able to choose a language.

<?php
/**
 * Add language column in ddbb
 */
function armys_update_7007(&$sandbox) {
  $spec = [
        'type' => 'varchar',
        'length' => 32,
        'not null' => FALSE,
  ];
  db_add_field('armys','language', $spec);
}

/**
 * Add language field
 */
function armys_update_7008(&$sandbox) {

  $field = [
      'field_name' => 'language',
      'type' => 'text',
      'entity_types' => array('army'),
  ];
  $field = field_create_field($field);

  $instance = [
    'field_name' => 'language',
    'entity_type' => 'army',
    'label' => 'Language',
    'bundle' => 'army',
    'required' => false,
  ];
  field_create_instance($instance);
}

?>

But I get this error

Failed: FieldException: Attempt to create field name <em class="placeholder">language</em> which is reserved by entity type <em class="placeholder">comment</em>. in field_create_field() (line 93 of /home/oskar/oskar/ProjectGCMS/gcms/www/modules/field/field.crud.inc).

I want to add a language field to my entity, but I don't how to make it, any suggestion.

Thanks.

Oskar