> \documentclass[12pt,openany]{mwbk}
> \usepackage{polski}
> \usepackage[utf8]{inputenc}
> \usepackage[T1]{fontenc}
> \usepackage{lipsum}
>
> \begin{document}
> \lipsum [1]
> żółty
>
> \end{document}
You don't need inputenc for UTF-8 with xelatex or lualatex, because both have
native support for UTF-8. But you should use fontspec instead of fontenc:
\documentclass[12pt,openany]{mwbk}
\usepackage{polski}
\usepackage{fontspec}
\usepackage{lipsum}
\begin{document}
\lipsum [1]
żółty
\end{document}
(Note, that this may fail if you're using a new LuaTeX, e.g., from TLcontrib,
but an old oberdiek.luatex.lua, e.g., from TeX Live 2012.)
Using xelatex you may also want to use
\usepackage{polyglossia}
\setdefaultlanguage{polish}
instead of \usepackage{polski}.
signature.asc
Description: This is a digitally signed message part.
