I solved this like that:
In bb_functions.php, I added one line (the middle one):
$dateR=date($GLOBALS['dateFormat'],$dfval);
$dateR=str_replace(array( date($GLOBALS['dateOnlyFormat']) , date($GLOBALS['dateOnlyFormat'],time()-86400) ), array($GLOBALS['l_today'],$GLOBALS['l_yesterday']),$dateR);
$dateR=str_replace($engMon,$months,$dateR);
To setup_options.php I added:
$dateOnlyFormat='j.n.Y';
And to my language file I added:
$l_today='dnes';
$l_yesterday='včera';
I think this is a neat way how to solve the issue - any comments?