I was getting recently a lot random crashes with xfce4-xkb-plugin. Also when I opened the plugin properties the greek layout variant text was all scrambled and ineligible. Having searched through the internet and bug trackers I couldn’t find something similar so I started suspecting that my setup was broken. A look in /usr/share/X11/xkb/rules/xorg.lst showed me that the greek layout was ‘gr’ and not ‘el’ as I had specified it.

You can get the greek layout code with:

grep -i 'greece' /usr/share/X11/xkb/rules/xorg.lst

and the keyboard variants with:

grep 'gr:' /usr/share/X11/xkb/rules/xorg.lst

so this is how my (correct) keyboard settings look:

Section "InputClass"
  Identifier       "Keyboard Defaults"
  MatchIsKeyboard  "yes"
  Option           "XkbLayout" "us, gr"
  Option           "XkbVariant" ",extended"
  Option           "XkbOptions" "grp:alt_shift_toggle, terminate:ctrl_alt_bksp,
lv3:ralt_switch_multikey, eurosign:e"
EndSection

(The lv3 multikey option is used to type ligatures see: http://shtrom.ssji.net/skb/xorg-ligatures.html)