that does not work. Installing japanese fonts will not work on either the opera browser nor the email app.
Furthermore, intalling Japanese fonts will not work on manaos browser either, despite reports that chinese web pages work in manaos with chinese fonts installed. Japanese encoding is a messy complicated affair, and I'm pretty sure that it is simply not possible to view Japanese on the 770 at this point.
However, it seems likely that there will be asian language support in the 2006 OS release.
I had a simmilar thread in the 2006 OS section, view it for some further info.