Posted: Sat Jun 13, 2009 7:53 am
Japanese Mode (Microsoft's Global Input Method Editor [IME]) is for programs that aren't Unicode language based. It is required to install progams that don't use Unicode for their output so it can function properly (for menus, display text, etc.).
Also, it allows Japanese characters to be your input on any application, not just those that naturally support it. This allows you to enter Japan mode and type in Japanese (assuming you've set the input associated to it to Japanese instead of English). For example, changing the display on Microsoft Word to Japanese and having your input be in Japanese, not English.
I'm surprise you didn't notice you were in JPN Mode the entire time. The "/" and "\" symbols become "¥" which annoys the fuck out of me. There's also some programs that look to see what Language you're in to determine which installer to run. Apple, for example, will do that on their updates. For awhile I had to deal with a Korean iTunes T.T
Edit: Though I suppose it is a non-issue if you have the Japan mode still output normally on programs that are Unicode-based languages and don't enable Japanese input. Not sure if that would leave everything alone except just the programs that require the Japanese IME (pretty sure it still does the two above problems).
Edit2: While I'm at it, I have no idea what the Unix-based OS equivalent would be. For Ubuntu, I THINK it is as simple as adding the language support and merely booting into Japanese Ubuntu but not sure. And not sure how to do some of this through WiNE for running Visual Novels or other Windows-based programs that you'd need this for (assuming they don't install cleanly into Ubuntu).