I was struggling to have the multi-language working in the description field as well until I realized that this field might not have a tag control.
My solution for this:
edit nggfunctions.php (nextgen-gallery root)
around line 489
replace
// description can contain HTML tags
$galleries[$key]->galdesc = html_entity_decode ( stripslashes($galleries[$key]->galdesc) ) ;
by this:
// description can contain HTML tags
$galleries[$key]->galdesc = html_entity_decode ( nggGallery::i18n(stripslashes($galleries[$key]->galdesc) ) ) ;
Now you can insert the description text by using the tag
[:xx] as you like/need
Ricardo
