Here's the long story:
I'm tring to have my finder and some apps in french but I want to use some other apps in english (Final Cut, Photoshop, a bunch of pro video apps for work). I use mainly FCP and the thing is that if I open it in french, for some obscure reason it assumes (erroneously) that I must be in France and that I have an azerty keyboard... so all the keyboard shortcuts are changed. It gets absolutely unusable, even undo is changed for something other than command-z. So I tried language switcher, with my system and finder set in french. I ran the list of my apps and decided, app by app, which one I wanted in french and which one I wanted in english. I saved those settings every time, because I felt it would be dumb to have to launch language switcher first each time I wanted to launch an app. Then I launched Final Cut Pro (7.0.3). At first it looked like all went well, it was in english, my system was in french. But when I checked trough the menus, I found that all the menu items were written in english but the keyboard shortcuts associated with them were the french ones! I tried to get it back to work normal by switching the system back to english and doing the reverse process with Language Switcher. Didn't work. I tried three or for other things that didn't work either (and I won't detail them all... sorry). So I reverted to my last time machine backup.
I assume the .lproj trick you mention is essentially what Language switcher is doing, so that wouldn't do me much good. I really miss the way we could do it before leopard...
anyways, thanks for answering!