$tvString = 'mycustomtv_' . $langId . ',' . $tvString;
The YAMS MM rules are designed to replicate the standard document layout. So, they place the multilingual equivalents of the standard document template variables at the top of the page. If you want to override this then I think you’ll have to edit the yams.mm_rules.inc.php file. The simplest way is probably to attach the names of the custom tvs to the front of the $tvString variable that is passed to the mm_moveFieldsToTab function on line 141. So, assuming your tvs have the format mycustomtv_langId, then on line 140 you could do something like:
$tvString = 'mycustomtv_' . $langId . ',' . $tvString;