I needed the Russian-QWERTY keyboard that can be installed on OS 9 with the language kits additions, and pukku (on ricford.com) gave me the hint that makes it work.
Keyboard layouts (which appear under the flag menu) are stored as resources inside the following file:
/System -> Library -> Frameworks -> Carbon.framework -> Versions -> A -> Frameworks -> HIToolbox.framework -> Versions -> A -> Resources -> English.lproj -> Localized.rsrc
You need to replace English.lproj with the language of your system, ie French.lproj
Next, with resedit or resorcerer, open the source OS 9 keyboard file (ie Cyrillic-QWERTY in my case), and copy the KCHR resource. Paste it in the Localized.rsrc file, and voila!
PS: in order to have the Cyrillic language on OS X, you need to install it first on OS 9, which is explained in article #106484 on Apple's Knowledge Base.
[Editor's note: You're on your own with this hint; I haven't done anything with non-English languages on OS X unless "Canadian" counts!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2002032411502969