Adding this to the sabnzbd init script solves the problem:
[ -r /etc/default/locale ] && {
. /etc/default/locale;
[ -n "$LANG" ] && export LANG;
}
It is fixed in the standard debian package by using this in the file /etc/default/sabnzbd. For reference https://forums.sabnzbd.org/viewtopic.php?t=9788