No it still doesn't work.
I tested what will be printed without this setlocale... and it said Monday, when I added:
setlocale(LC_TIME, array('pl_PL.UTF-8','pl_PL@zloty','pl_PL','polish'));
It changed to PoniedziaĆek, which is a Polish translation of Monday, but the name of the month still is in English in osclass custom forms and and in dashboard (except for aforementioned cron).