Have you tried using utf-8 characters? Here'a list for Polish characters: http://webdesign.about.com/od/localization/l/blhtmlcodes-pl.htm
Ensure that you have <meta http-equiv="content-type" content="text/html;charset=utf-8" />
as a meta tag and you should be set. Plop this into your header, and it'll all be gravy. For "ęą" use ę
and ą
respectively
EDIT:
Unrelated to the question, but if you are using Wordpress for a multi-lingual site check out the _e
Wordpress function. I've used it before and it's treated me well http://codex.wordpress.org/Function_Reference/_e