comma separated decimal numbers in pgf-pie

Package pgf-pie provides two hooks for printing the numbers: before number and after number. They define macros \beforenumber and \afternumber. The number is set as macro \p between the two hooks. This can be used to smuggle in a macro, which grabs the number and passes it to a more powerful package for setting numbers, package siunitx, for example:

\documentclass{article}

\usepackage{pgf-pie}
\usepackage{siunitx}
\sisetup{
  output-decimal-marker={,},
  group-separator={\,},
}
\def\ScanPercentage#1\afternumber{\SI{#1}{\percent}}

\begin{document}
  \begin{tikzpicture}
    \pie[
      before number=\ScanPercentage,
      after number={},
    ]{20.1/xxx,79.9/yyy}
  \end{tikzpicture}
\end{document}