Translate CPT selection in posts with Polylang

Hi there.
I have a custom post type and added to posts a meta box where I can select the CPT. This is my code:

    // Selection for exhibitor on admin posts page
    $meta_boxes[] = [
      'title'      => esc_html__( 'Exhibitor', 'ex' ),
      'id'         => 'exhibitor',
      'post_types' => ['post'],
      'context'    => 'side',
      'priority'   => 'low',
      'autosave'   => true,
      'fields'     => [
        [
          'name'        => esc_html__( 'Assigned exhibitor', 'ex' ),
          'id'          => 'exhibitor',
          'type'        => 'post',
          'post_type'   => 'exhibitor',
          'field_type'  => 'select_advanced',
          'placeholder' => esc_html__( 'Select an exhibitor', 'ex' ),
          'query_args'  => array(
            'post_status'    => 'publish',
            'posts_per_page' => - 1,
          ),
          'ajax' => true
        ]
      ]
    ];

I do also use Polylang to translate the content.
I want to make sure the selected CPT (in my case the exhibitor) it is "translated". It is not working out of the box. I found this topic https://wordpress.org/support/topic/metabox-and-polyang/ which points me to https://wpml.org/documentation/support/language-configuration-files/.
I created a wpml-config.xml file with this content:

<wpml-config>

  <custom-fields>
    <custom-field action="translate">exhibitor</custom-field>
  </custom-fields>

</wpml-config>

But, I do not get the field translated.
There are several questions:
1.) Is the file correct?
2.) Where Do I habe to store the file? In the plugin directory, which is setting up the meta box or, in the theme directory?
Thank you for your reply!