multilingualpress-cross-post-type-connection.php

<?php
/**
 * Filter the post search arguments allow for cross-post-type connections in MultilingualPress.
 *
 * MultilingualPress (MLP) connects translated posts and pages, but by default doesn't connection across post types.
 * This changes that behavior by allowing other post types to be chosen in the interface. No other modifications are
 * needed to achieve this, because MLP uses the post_id and doesn't care about post_type outside of the search query.
 *
 * @param array $args the WP_Term_Query arguments used.
 * @return array
 */
add_filter( 'multilingualpress.remote_post_search_arguments', function( $args ) {

	// Change this array to the post types you want to connect.
	$translated_post_types = array( 'post', 'page', 'product' );

	// Make sure we have a valid $args['post_type'] value
	// and that we aren't removing the current post type.
	if ( isset( $args['post_type'] )
		&& is_string( $args['post_type'] )
		&& in_array( $args['post_type'], $translated_post_types, true ) ) {
			$args['post_type'] = $translated_post_types;
    }

	return $args;
}, 100, 1 );