What I did was:

- First download the source of ukeyboard ("apt-get source ukeyboard" in scratchbox, or find it in the garage)
- Open a keyboard definition (.def) that is the most similar to the one you are trying to produce
- Change the definition to match the new language and save it as a new .def file
- build ukeyboard " ./configure; make "
- use the vkb_compiler to compile your new definition to a .vkb file
- copy the .vkb file to /usr/share/ukeyboard or /usr/share/keyboards (not sure, I have my file in both )
- test and make sure it works
- submit it to the ukeyboard maintainer

The procedure is the same for hardware keyboards, but it's a different file and the language code needs to get compiled into the ukeyboard program, so you can only test it on the N900 by replacing some other language definition in /usr/share/X11/xkb/symbols/nokia_vndr/ukeyboard