External filebrowsers and filemanagers
does not support this function at the moment.
Normally all subsites and second languages are working with the userfiles folder of the main installation. The filebrowser will upload files to that folder and its subfolders, and the dialogs of the editor connects to that folders (for example for insert images).
Now it may be, that subsites or second languages have their own authors, who wants to manage their own users files (images, downloads, multimedia files) in a separate userfiles folder.
This you can do by create an userfiles folder and its subfolders (or copy them from the main installation) in a subsite or second language, and make all folders writable.
After that, the corresponding subsite or second language works with these own userfiles folders, and the author has no more access by backend to the userfiles folder of the main installation.
This can be handled differently for each subsite or second languages.