How to configure zenphoto to get polish characters in archive or anywhere else on site?

It's fresh install, nothing is user inputed so I assume it's zenphoto's thing.

Screenshot: http://scr.hu/1n9b/ec4k0

It is actually a PHP (=Server) thing. Simply said Zenphoto uses the PHP date functions to display the months based on the locale set.

Try the encoding settings on the options, maybe your server is not UTF-8 but a specicic Polish based encoding setting.

Screen shots are of little use to us for things like this. There are multiple causes of characters not being displayed correctly. This "looks" like it is probably the [b]default[/b] theme, but we can't even confirm that much less guess what you have the site character set option set to.

Other issues are the character set of your file system and what character set is needed for URIs. Setup will have discussed these issues with you if you peruse the warnings. Suffice it to say if you have not made the recommended 'charset_tést' file there is no way for Zenphoto to know what it should be doing.