I use el_GR.UTF-8 which does not group thousands. I changed /usr/share/i18n/locales/el_GR, updated:
sudo locale-gen el_GR
sudo locale-gen el_GR.UTF-8
sudo update-locale LANG=el_GR.UTF-8
but still printf "%'.1f\n" 1000 produces 1000,0 instead of 1.000,0