Hi jjk,
In Linux you can do same things different ways.
1) edit your /etc/X11/xorg.conf.d/40-Keyboard.conf (you need root rights):
Before line "EndSection" add this line:
Option "XkbOptions" "compose:ralt"
I think you have not other lines with "XkbOptions"?
Your compose key will be RightAlt, but you need to restart X (reboot system or logout/login to system).
2) Or you can open your terminal and type:
setxkbmap -option "compose:ralt"
And your compose key will be RightAlt immediately. But system forget this after reboot, and you need to put this option to your ~/.config/openbox/autostart.sh (you can find "Auto started applications" in the SliTaz menu for it).
You can set other key up for Compose key. Citation from /usr/share/X11/xkb/rules/base.lst:
Compose key Compose key position
compose:ralt Right Alt
compose:lwin Left Win
compose:rwin Right Win
compose:menu Menu
compose:lctrl Left Ctrl
compose:rctrl Right Ctrl
compose:caps Caps Lock
compose:102 <Less/Greater>
compose:paus Pause
compose:prsc PrtSc
compose:sclk Scroll Lock
Good luck!