I don't think you need a localized version of OS X, you just need to turn on the Polish language as an option in the Input Type of International System Preferences. You can then switch between English and Polish (or any other language) as an input method on a per-application basis. Keyboard Viewer will help you work out what the keystrokes are (obviously some of the labels on your keys will not be useful when you switch to another language).
I don't really understand it but if you intend sharing your documents with someone else who is not using a Mac, then you need to be sure that you are using Unicode characters. There are some posts on this in the Pages Discussions.
I hope that helps