This bug has been reported to the Debian BTS 4 years ago by Sean Neakums regarding XFree86 4.2.1 and is still valid for Xorg 7.2.
Whether I start Xnest with no options, with -kb or with +kb, I cannot
seem to get the XKEYBOARD extension. It's not listed in xdpyinfo
output. According to the submitter, it used to work with XFree86 4.1.0.
This keymap problem makes Xnest pretty hard to use for keyboards
all over the world other than US.
It seems to work fine with Xephyr +kb. I understand that Xephyr should be preferred to Xnest, so this bug might never be closed. But at least it will be known.
Sorry about the phenomenal bug spam, guys. Adding xorg-team@ to the QA contact so bugs don't get lost in future.
XKB was disabled with
Author: Daniel Stone <firstname.lastname@example.org>
Date: Mon Oct 23 02:51:52 2006 +0300
Xnest: disable XKB, reshuffle code
Disable XKB, as we can't yet use it; move Composite disabling to
ddxInitGlobals, along with XKB.
re-enabled now, pushed as 2ddbfd345786aa39b6ccaed82a1ca5c145284ee3.
Tested with a gnome-session, and it seems to work.