Bug#529582: TeX ligatures aren't set up by default.

[Adam Buchbinder <adam.buchbinder@gmail.com>]

This isn't a bug, but it's definitely a recurring problem that people
run into when moving to XeTeX or XeLaTeX.

XeTeX and XeLaTeX don't set up TeX-based ligatures by default, possibly
because they support direct UTF-8 input. To get the ligatures, enter the
following after your \usepackage{fontspec}:

\defaultfontfeatures{Mapping=tex-text}

However, if you're still using the Computer Modern fonts, this won't
work, since it will only map fonts loaded after the fontspec inclusion.
To ensure that ligatures are enabled for Computer Modern fonts, add the
cm-default option to fontspec:

\usepackage[cm-default]{fontspec}
\defaultfontfeatures{Mapping=tex-text}

See here for more information:

http://www.tug.org/pipermail/xetex/2007-October/007535.html

Also, I don't know anything about the 'polski' package; it's possible
that it might not be XeLaTeX-compatible (on the other hand, it might
be). If you're looking to support other languages, and you need
something that basic Unicode support doesn't provide, the polyglossia
package might be helpful, though it requires a newer version of XeLaTeX
than Debian ships.

Adam Buchbinder