I've experienced the same problem before I switched to the Swiss French keyboard. AltGr works in fr_CH-latin1 (lang EN).

Hi,
If you are using Xorg, please edit your file /etc/X11/xorg.conf and look for the InputDevice section.

In my case I had to add this line (which was not present initially) to be able to use "alt-gr":

        Option      "XkbRules"   "xorg"

You should have something similar to my /etc/X11/xorg.conf:

Section "InputDevice"
        Identifier  "Keyboard0"
        Driver      "kbd"      
        Option      "XkbRules"   "xorg"
        Option      "XkbLayout"  "ch"  
        Option      "XkbVariant" "fr"  
EndSection                             

Hope it helps.
Cheers
Stephane