The German translation of the useradd man page contains a typo descriping the "-g" option. USERGROUPS_ENAB is defined in /etc/login.defs, not /etc/default/useradd. Wrong: Falls nicht definiert, hängt das Verhalten von useradd von der Variable USERGROUPS_ENAB in /etc/default/useradd ab. Correct (and mentioned in other language versions of the man page): Falls nicht definiert, hängt das Verhalten von useradd von der Variable USERGROUPS_ENAB in /etc/login.defs ab.
"Typo" means someone typed the wrong character.
thanks, i've sent a fix upstream for it now