Where?.
Translate descriptions in tickethistorylib.php.
Also some useful things in ticketlib.php - example to polish:
$slist = array('open' => 'otwarte', 'closed' => 'zamkni?te', 'fixed' => 'naprawione', 'analyzed' => 'przeanalizowane');
also priorities:
$vlist['priority'] = array('s', array('niski', '?redni', 'wysoki', 'pilny'));