By default, you can use voice search in Japanese with the new version of Google’s Mobile App for iPhone only if your iPhone language settings is also set to Japanese. If the iPhone is set for any other language, the voice search options will be English (Australia/India/UK/US) or Chinese (for some reason, Japanese isn’t on this list).
To enable japanese voice search, change your iPhone language to Japanese (Settings > General > International > Language). If you are not familiar with Japanese characters, do this with someone who is. Then, run Google Mobile App, change Voice Search’s language setting to Japanese (日本語). Finally, reset iPhone’s language settings back to your language.
Your Google Mobile App voice search setting should appear as in the picture above, without any language selected, and you’ll be able to do voice searches in Japanese.
I think this could be very useful for foreigners living in Japan who are capable of searching for addresses, places, names and so on, or even for those who are not familiar with kanji/Japanese characters.