As mentioned above, it can all be solved with AutoHotkey script. Both alt+s and alt+n is fixed by running a following script in AutoHotkey:
Not sure, who is the original author. but credit goes to him/her. I've just added the alt+n workaraound.
RAlt & s::
If GetKeyState("Shift","P")
Send {Text}Ś
else
Send {Text}ś
return
RAlt & n::
If GetKeyState("Shift","P")
Send {Text}Ń
else
Send {Text}ń
return