I have MSLS set up with two languages (“us” and “fr_FR”) and this worked for me:
// to get the French version of an English post
// ($post_id = id of the post in English)
$translation_post_id = ( get_option('msls_'.$post_id) )["fr_FR"];
// to get the English version of a French post
// ($post_id = id of the post in French)
$translation_post_id = ( get_option('msls_'.$post_id) )["us"];