Koji Arai put together a Ruby Tk manual in Japanese. This manual looks 
like it is very good (from the code samples). Has anyone translated it 
into English, or is anyone working on an English translation, or is 
anyone planning to do an English translation?

The Japanese language document is available through the RAA:

http://raa.ruby-lang.org/list.rhtml?name=rubytk-man

Regards,

Mark