Thanks for your interesting of my problem!
I'm not sure if I understood you right, but I'm not using old polish language files. They are not working good with bootstrap edition.
At first I was just using original english language with some changes like european date showing, but i didn't change anything like charset, because everything has been shown as it should.
As earlier I supposed that non-standard character conversion doesn't work with polish letters on english language, I've duplicated it with few changes to being recognised by oscommerce as polish, but it didn't help.
I'm using notepad++ and it shows that my lng files are coded in UTF-8 (without BOM).
Also, in PHPMyAdmin every letter is shown as it should be.
Please, look at my screen. Those are all polish letters. I've changed products_name directly in the database. As you can see, only ó=>o is working well. http://i.imgur.com/A16VoRH.jpg