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!]

