> Can someone please explain why is it that if I compile the document:
>
> \documentclass{article}
> \usepackage{fontspec,lmodern}
> \usepackage[T1]{fontenc}
> \begin{document}
> «A»
> \end{document}
>
> what I get is
>
> ńAż
>
> instead of
>
> «A» ?
Because you're using fontenc. Don't. Just let fontspec do the job.
Arthur