The whole thing just became a moot point. I installed the developer tools beta, and there's no sign of any way to change input languages. There's not even any space on the keyboard for a button to cycle between input languages like the iPhone has, which
leads me to believe that WP7 will be able to input text only in one language, probably linked to the UI language.
This makes it impossible for me to develop a usable Japanese dictionary. It also means that I definitely won't be getting a WP7 device (even if they are released in Japan, which I'm not too sure of). For me a phone is only an option if: 1. I can buy one
in Japan yet still use English UI, and 2. even with English UI, I can still read and input Japanese text.
Its ability to do that is the primary reason why I picked an iPhone when I last got a new phone. WM6.x actually fails on both these criteria. I was hoping WP7 would do better but it seems unlikely at this point.