It seems that this is made possible by dual names of the folders/apps. One is the visible name that you see in the Finder when you're logged in as a non-English user. This name is also displayed at the top of the Get Info window, but is only changeble from the Finder (if changeable anywhere).
The other name is the "real" name, shown in the "Name & extension" part of the info window. This "real" English name is used by the system to recognize the files and folders. Fine, soo far.
PROBLEM: After installing Jaguar (clean & archived install with kept home folder), the visible names of folders in my old pre-Jaguar user folder stayed the same, that is with English names.
Read the rest of the article for more on the problem and pre- and post-install solutions.
Now - if I created a new user in Jaguar, all folders of that user were named in my preferred language, even the Desktop folder and the Library folder. But when I tried changing the names of my old Home folder folders (although possible, except for Desktop), also the "real" name will change, which of course is no good. For example, applications can't find their preferences, since the "real name" of Library folder has changed. So - it seems that my old folders didn't have this dual names-possibility.
CONCLUSION: What I have learned is that if you want to "internationalize" your home folder, you should leave the "keep network settings and home folder" box (maybe not a correct translation) unchecked. Then, after installantion, create a new user with the same name as the old one, and copy the contents of the old sub-folders (Desktop, Library and so on) from your old user folder (now located in /Previous system -> Previous system 1 -> your_username) to the new user folder.
SOLUTION: If you made the same mistake as I did, you can do this change afterwards too:
- Create a new, temorary user.
- Log in as root.
- Change the owner of all the eight preinstalled folders in the temporary user folder to your username, with the Get info command.
- Do the copy-procedure like above (from subfolders in your old user folder to subfolders in the temporary one).
- Move the eight subfolders from the temorary user folder to your own.
- Log out from root, and log in to your own account. Now even your home folder should be internationalized.

