Hello Gebana,
The problem is that Apple only provides text-to-speech in English. To make VoiceOver work in Japanese, you will need a Japanese text-to-speech engine. There are several around, but the most suitable one would appear to come free with a shareware notes program, VoiceNotes. You can get this program via Version Tracker. Here's the link:
http://www.versiontracker.com/dyn/moreinfo/macosx/23720
The shareware fee is $29.00.
I can't test it out for you as I don't speak Japanese…
Once you have the Japanese TTS installed, you should then set the system voice to that name (in the Speech control pane of System Preferences) and the various speech options of VoiceOver to use the same voice via the VoiceOver utility.
The VoiceOver utility is also the key to making the non-localised strings sound OK, by using its Pronunciation tab. For example, you could tell VO to say the Japanese phrase for "interacting with" when it encounters the English words "interacting with". This will prevent the "Japanese voice speaking Engrish very badry" syndrome.
The next problem is the "input method", otherwise known as the keyboard layout. I assume you'll be needing to type mostly in Japanese, so you should have the keyboard set that way. However, VoiceOver, when the VO keys are held down, interprets the keyboard as the English layout (British or US). So when navigating, you'll have to remember the English layout, and when typing a document, the Japanese!
This presents a problem when setting up the pronunciation dictionary I referred to earlier. I recommend that you create an English-language user as well as your main Japanese-language user. Then you can switch from one to the other using Fast User Switching.
Prepare a list of VoiceOver terms in English that you wish to have read in Japanese, under the English user-id, then save the document into the Shared folder. Change to the Japanese user, open the shared document, and clipboard a phrase at a time into the VoiceOver Utility Pronunciation Dictionary. Then type in the Japanese equivalent.
Obviously, your list of English terms will be an evolving document as you find more annoying phrases!
My wife has done this with a French voice and it works pretty well on the whole.
Let us know how you get on!
Archie