\setuplabeltext[de][aufgabe={Aufgabe ,}]

\define[1]\TaskCommand

  {\doifsomething{#1}{: #1}}

\definehead[thetask][section]

\setuphead

  [thetask]

  [style=sansbold,

   bodypartlabel=aufgabe,

   distance=0pt,

   deeptextcommand=\TaskCommand]

\definecomplexorsimpleempty\task

\def\complextask[#1]{\thetask{#1}}

\mainlanguage[de]