@Doitsu...You are correct in saying that both the TOC names and file names in the Book Browser are not dispalying with Polish characters. I derive these file names directly from the ebook file as utf-8. But unfortunately -- due to
pythonissue 27344 -- zip file names on Windows can only use DOS Latin and not utf8.
And since I derive both the xhtml file names and content.xhtml toc items in the same way I am unable to fix these problems at the moment. Regarding the content.xhtml toc names, I'm still looking into how I can generate the Polish names for the toc. One way to solve both the toc and file names problem would perhaps be -- as KevinH has suggested -- to generate the epub zip file by simply using the epub_zip_up_book_contents(ebook_path, epub_filepath) in Sigil's plugin utilities. This will be a major change requiring much testing, so I think I'll do that after the plugin has settled a bit regarding other more minor errors.