Earlier quoted context omitted.
What browser are you using? I can see all the characters. They have specified 'sans-serif' (which most of the times means Helvetica).
I'm on Chrome 13 and yes, I see all characters too, but Ż, ž, č, ą and ę are displayed in a fall-back sans-serif font. All others using Istok Web, as defined in the CSS. It doesn't look good.
font-family: 'Istok Web' sans-serif;
should be: font-family: 'Istok Web', sans-serif;
Firefox 7 ignore the above declaration showing the page in the default font. I have forced 'Istok Web' with firebug and yes it looks ugly.