Finally I started converting my workflow of day-job I do on my
home machine from darcs to git, so I started running cvs2git
over ssh slurping from my office environment (sloooooooooow).
While I was watching the import going, I started up gitk on the
halfway imported repository, and it showed Japanese characters
in the source correctly without any special configuration (the
only thing I have is LC_CTYPE=ja_JP). Very often GUIish
software would not work properly for me with Japanese
characters, and this was a pleasant surprise.
The credit is probably owed more to tk, not something you do
special in gitk, but nevertheless I am impressed and happy ;-).