i18nviews_handler_field_term_description depends upon a term id being available:

$tid = (isset($value->term_data_tid)) ? $value->term_data_tid : $value->tid;
        $values[$key]->term_data_description = tt('taxonomy:term:'. $tid .':description', $value->term_data_description);

There is no guarantee that a tid will be available, which makes the tt fail to translate. As a workaround, I've added an excluded term id field, but clearly there's a problem with this handler's approach.