The Cacko rom removes all the files needed for inputting and displaying Japanese, they also remove the dictionary files used by sljisho and the translator. So, no you won't be able to input Japanese after installing the Cacko rom.
If you want to have English menu's and keep all the Japanese reading/input ability of the C860 then I recommend converting the Zaurus yourself. It is actually an easy thing to do. I wrote a script that does almost all ofthe work for you - it has been posted here several times already so I won't post it again, a quick search should find it easily. You will need to have a terminal app installed and run the script as root. After that restart qtopia and the menus etc will be in English
Stu