Guide: Adding Japanese as a language and using it in-game.
BEFORE YOU DO ANYTHING
I strongly suggest making an account at Adventures in JMS first as that is the most popular website for English-speaking JMS players and if you need to seek help, you’ll certainly find it there. There is also a JMS section at BasilMarket.
ADDING JAPANESE AS A LANGUAGE AND USING IT IN-GAME
Disclaimer: If you feel that adding Japanese as a language has affected your computer in any negative way, it’s not my fault.
I am writing this for Windows XP users, but the steps are still incredibly similar in Windows Vista. However, if you’d rather view a guide for adding Japanese as a language that’s specifically aimed towards Windows Vista users, there is an excellent one here, but it does not tell you how to switch between Japanese and English and type Japanese in-game.
You will need a Japanese language pack to view and use Japanese characters. If you have the language pack installed, you should be able to see this: こんにちは。 If you only see boxes, you will need to install the language pack. You’ll be prompted to install the language while following this guide, so insert your Windows disk to install them when required. If you don’t have a Windows disk, follow the guide here.
Here is a summary if you don’t feel like reading: Add Japanese as a language. Use Left Alt + Shift to switch between Japanese and English as this is the Windows default. After hitting Alt + Shift you are still using direct input, not Japanese characters, so therefore you must also in-game hit Alt + ` to begin typing with Japanese characters. Enter romaji and it’ll come up as hiragana. Hit space to display a list of katakana and kanji characters you can change to.
1. Go to your Control Panel. This can be accessed by going to “Start” and selecting “Control Panel”.
2. Go to “Regional And Language Options” (If you do not see this icon, switch to classic view using the options at the left.)
3. A window will appear. Go to the “Languages” tab and select “Details.”
4. A new window will open similar to this one. To add Japanese, click “Add“.
5. Select Japanese from the list of languages and then click “OK“.
6. The installed services should now look like as pictured. Now you have Japanese as a language you can use on your computer, but to be able to easily switch between Japanese and English, choose “Key Settings“.
7. As you can see, by default, you can switch between languages by simply hitting Left Alt + Shift. You can do this in game too. You can leave it like this if you want! However if you’d prefer to add a better shortcut, select that language and click “Change Key Sequence” and add in your own better shortcut there (that’s just one little box so I won’t bother with a screenshot). Once that is done, click “OK” to get back to the main window.
Click apply to save your settings and then OK. You’re done with this part!
8. After clicking apply, you can change your language bar settings. The language bar isn’t a necessity for switching between English and Japanese, because you can use keyboard shortcuts for that, but this step will be useful anyway as you’ll see why you need to use Alt + ` in-game to type in Japanese after changing languages. If the language bar does not show up on your taskbar right away, right click it, go to “Toolbars” and check “Language Bar.”
Something like this should appear on your task bar. 
Click the “EN” and choose Japanese, or simply use Alt + Shift.
The bar will now look like this (You may need to right click it again and choose “Adjust the Language band position” to see all the icons).
9. Notice the “A”. Even though you’ve switched to Japanese, you’re still inputting with English characters (direct input). To change this simply click the “A” and choose Hiragana. The ‘A’ symbol will then become an ‘あ’.
However! You can also do this with a simple keyboard shortcut, Alt + `, and that is the shortcut you need to use in game to switch between direct input and hiragana. ` is the key beside 1 and above Tab.
You can now type Japanese in-game. :] Enter romaji and it’ll come up as hiragana. Hit space to display a list of katakana and kanji characters you can change to.
SUMMARY
Use Left Alt + Shift to switch to Japanese.
Use Alt + ` to switch from our alphabet to Japanese characters.
♬☁☀☂☆ 