I have installed the english version of wordpress and then changed one setting in wp-config.php file:
as a result I have most of the wordpress menu in polish.
In wordpress admin's inputs and textfields I'm using all letters ("ąęśćżł..." also) and everything works well. Even in other plugins (where I get labels in english, so I assume they do not have any special polish version :) ) it all works well.
The only problem I have is in Weekly Class Schedule plugin. In all inputs/textfields in this plugin, when I use those non-english characters and save changes, I get all those characters changed to "?".
It's possible that there is some charset mismatch.. (but where?... and how to change it?..) .. Maybe it's during saving to database? Maybe fields have some setting to accept only Latin characters and ignore Latin Extended? ....