Bug#413248: installation-reports: (etch) strangeness with non-ASCII-character filenames on vfat partition
> It could also be that there really is an issue with the display of VFAT
> filenames if UTF-8 is used, but that would not be my first guess.
> Anyway, I doubt this would be an installer issue as there is no real way
> for the installer to determine the correct settings.
VFAT stores filenames in UTF-16, the driver has to convert this to something byte-orientated for processes/users to see.
i suspect the problem is that the driver is still defaulting to converting to a legacy encoding while the user interface is now UTF-8
can you try adding iocharset=UTF-8 to the mount options for the partitions and telling us if that makes stuff behave as you expect?
if so then imo this option should be set by the installer when generating fstab as afaict newly installed debian systems are completely utf-8 based nowadays.