On a german keyboard, the [AltGR] + [+] combo to get "~" doesn't work.
To work around this use the following workaround.
It maps "Scroll-Lock" (code 0x4E, "Rollen" in German) permanently to the tilde.
Look for a file in your home directory called ".Xmodmap". Keep a backup of that file, just in case.
Use an editor of your choice to change the line starting with "keycode
0x4E", if it is already present, or add it:
keycode 0x4E = asciitilde
Log out of X and log back in again (or use $ xmodmap ~/.Xmodmap).
You should be able to use scroll lock as the console key. If you want to play around with the xkeycaps tool (http://www.jwz.org/xkeycaps/) it is possible to write a wrapper around q3a that remaps the key:
[/usr/local/bin/q3german.sh]
#!/bin/sh
ps2speed 200
cd /usr/local/games/q3demoTEST
xmodmap -e "keycode 49 = asciitilde"
sudo ./linuxquake3 $*
xmodmap -e "keycode 49 = asciicircum"
...
This maps "the key beneath ESC" (which is "^" on german keyboards) to "~" and everything works as expected (except that you can't type "^" anymore...).