Extraordinary

Exactly what I was looking for ! Works great with the latest version of WordPress (5.3)

No need for shortcode! To obtain a featured image that loads automatically according to the language of the plugin qtranslate-x, you can add in the file functions.php:

add_filter( 'kdmfi_featured_images', function( $featured_images ) {
	$args = array(
	  'id' => 'featured-image-en',
	  'desc' => 'English version of the feature image.',
	  'label_name' => 'English featured image',
	  'label_set' => 'Set english featured image',
	  'label_remove' => 'Remove english featured image',
	  'label_use' => 'Set english featured image',
	  'post_type' => array( 'page', 'post', 'portfolio', 'app' ),
	);
  
	$featured_images[] = $args;
  
	return $featured_images;
  });

  add_filter( 'get_post_metadata', 'wdm_change_featured_image', 10, 4 );
function wdm_change_featured_image( $feature_image_id = null, $post_id, $meta_key, $single ) {
    if ( $meta_key == '_thumbnail_id' ) {
         if(function_exists('qtranxf_getLanguage')) {
        if (qtranxf_getLanguage() == 'en' && kdmfi_has_featured_image( 'featured-image-en', $post_id )) {
            $feature_image_id = kdmfi_get_featured_image_id( 'featured-image-en', $post_id );
        }
    }
    }
  return $feature_image_id;
}