`xlib:modifier-mapping` not listing AltGr as modifier some non US layouts

[Phundrak]

As reported in https://github.com/stumpwm/stumpwm/issues/936, the function xlib:modifier-mapping does not list AltGr (code 108) as a modifier when either a French or the default Spanish layout is set. More could be affected by this issue.

Steps to reproduce:

• Set keyboard to us with setxkbmap
• Execute the following code in SBCL:

  (ql:quickload :clx)
  (let ((display (xlib:open-display "")))
  (xlib:modifier-mapping display))

• See the code 108 corresponding to AltGr is here.
• Set keyboard to fr or es with setxkbmap (some if not all variants of fr are also affected)
• Execute the same code in SBCL
• The code 108 does not appear in the output

I am very unfamiliar with Xorg programming, so I have no idea where this issue might come from. Any idea?