% https://tex.stackexchange.com/q/308883/
\documentclass{article}
\usepackage{xsim,siunitx}
\xsimsetup{goal-print={\num{#1}}}
\sisetup{output-decimal-marker={,}}

\begin{document}

\begin{exercise}[points=2.5]
  foo
\end{exercise}

\end{document}