Normally, all the translations are done by Nextgen Gallery using WPML API. And Polylang just provides the same functions as WPML :). Maybe this one has been forgotten.
It should also work by doing:
<?php echo html_entity_decode ( nggGallery::i18n( stripslashes($gallery->title), 'gal_' . $gallery->gid . '_title') ) ; ?>This is the standard way Nextgen gallery uses to translate strings. If it works, maybe you can propose the patch to the authors as it should work with other multilingual plugins too. The solution with 'pll__' does work only with Polylang.