The device supports Unicode sure, but FTP (only one of the available protocols) was written before Unicode existed. It supports ASCII only, and there is no real way around that.
ohh and to confirm was
Using the window explorer (XP & Vista), I can't read and transfer the non-english files/directories
supposed to be
Using the window explorer (XP & Vista), I can read and transfer the non-english files/directories
Because putting the FTP after it saying but then you can't, and trying to read via FTP (implying files are already there), makes me think that was a typo and my responses have been based on that assumption (we all know what happens when you assume).
Use SMB/CIFS or NFS.