For me there are two kinds of Japanese support:
The first is the filesystem - does it support Japanese filenames. It can do, but not by default on SME6.0.1 You need to modify smb.conf here is the top of my smb.conf
case sensitive = no
#character set = ISO8859-1
#client code page = 850
client code page = 932
coding system = EUC
character set =
The second is support of Japanese emails. SME Server 6.0.1 and 6.5 beta both fail here

However the good news is that the community have a solution in the form of a HowTo to install Horde 3 and IMP4 to 6.x which can be found in the forums.
I hope this points you in the right direction - just post again if there are problems.