I thought it will be a piece of cake - I started with titlesec, but it ended up with:
Non standard sectioning command detected(titlesec) Using default spacing and no format.
Then I started digging some other topics, but eventually none of them helped me with this problem. I guess the issue lies within the mwrep class which I'm using, but I'm clueless what hack am I supposed to use. I would greatly appreciate any help. Here's the example:
\documentclass[12pt,a4paper]{mwrep}
\usepackage[MeX]{polski}
\usepackage{fontspec}
\usepackage[]{unicode-math}
\setmainfont[Mapping=tex-text]{Times New Roman}
\setmathfont{XITS Math}
\defaultfontfeatures{Ligatures=TeX}
\usepackage{setspace}
\usepackage[top=2.5cm, bottom=2.5cm, left=3.5cm, right=2.5cm]{geometry}
\usepackage{titlesec}
\titleformat{\chapter}[display]
{\normalfont\huge\bfseries}{\chaptertitlename\ \thechapter}{20pt}{\Huge}
\titlespacing*{\chapter}{0pt}{-50pt}{100pt}
\doublespacing
\begin{document}
\chapter{Litwo, Ojczyzno moja!}
Ty jeste\'s jak zdrowie\ldots
\end{document}
