I found a solutions.
The gallery title and description are placed in the database like that in plugins\nextgen-gallery\admin\manage.php:
$wpdb->query( $wpdb->prepare ("UPDATE $wpdb->nggallery SET title= '%s', slug= '%s' WHERE gid = %d", esc_attr($_POST['title']), $slug, $this->gid) );
and
$wpdb->query( $wpdb->prepare ("UPDATE $wpdb->nggallery SET galdesc= '%s' WHERE gid = %d", esc_attr( $_POST['gallerydesc'] ), $this->gid) );
I removed the esc_attr(). This makes that the codes for translation (<!–:–>) will be correctly placed in the database.
For showing the title and description on my gallery page, I used the following code:
<h1><?php echo html_entity_decode ( nggGallery::i18n( stripslashes($gallery->title))); ?></h1>
<p><?php echo html_entity_decode ( nggGallery::i18n( stripslashes($gallery->description))); ?></p>
If you would just use echo $gallery->description, then you’d see the complete description with all the translations.