Here is how to to switch back to a period symbol:
dpkg-reconfigure keyboard-configurationcannot do it for you but thanks to this bug fix it will also not erase your changes.
sudo udevadm trigger --subsystem-match=input --action=change
then restart Xorg
man xkeyboard-config and this Debian X Strike Force page for more.
EDIT: this bug is fixed in the upstream repository: https://bugs.freedesktop.org/show_bug.cgi?id=74274