The Apple IIGS ToolBox allows programmers to do some interesting things.
You can, for example, redefine the keyboard layout by patching certain calls
to the ToolBox.
To create new character sets, you'll intercept the data coming from the
keyboard, translate it to whatever you want, and store it it the proper
place.
To learn how to do this, you'll need the Apple IIGS ToolBox Reference
Manual (two volumes, APDA order number K2BGST). This book describes the
ADB toolset that you would use to alter the keycodes. The Keyboard Micro
gets data from the ADB keyboard, then software is called to get that data
and send it to the $C000 area. Your software would read the keyboard micro,
and then translate the keycode that it gets back into an ASCII (or whatever)
type of code, storing it back into the $C000 vector.
Altering the character set is not possible in text mode, because the
character sets are in ROM (in the VGC custom IC) and can't be changed.
But there is a simple way to accomplish this: use the standard graphics
environment that the Apple IIGS provides. By using the Font Manager, you
can create your own customized fonts for any language and any type of
symbols. This system is also documented in the Apple IIGS ToolBox Reference
Manual.