I’ve come to a conclusion that the issue is a polylang issue rather than ACF issue. If one is using only “post__in” and “post_type” in wp_query, it shouldn’t make a difference in which language the request is made in as we has the IDs. Alternative solution would be to change ACF Core class-acf-field-relationship.php to iterate over every saved ID and map its content to a new array with “get_post”. “get_posts” with “post__in” does not work but “get_post” with single ID works.
