I'm using XeTeX because I want to use some fonts from my system. The next should work (I think).
I have this:
\documentclass[a4paper,11pt,spanish]{article}
\usepackage[margin=2.5cm]{geometry}
\usepackage{babel}
\font\reenie="ReenieBeanie"
\font\cuprum="Cuprum"
\renewcommand{\sffamily}{\reenie}
\renewcommand{\familydefault}{\cuprum}
\begin{document}
...
\end{document}
The compilation fails, although I get the document with the new font and some weird symbols arround it. What am I doing wrong?