I'd like to target our game for different languages such as English, German, French, Spanish, and Japanese. We're using TrueType fonts for our game, but I'm not sure if they're used for the Japanese character set. Any ideas on how to use custom fonts for Japanese? I've noticed some game on the iOS App Store that just uses a generic Arial/Helvetica font for their Japanese localizations, but I'd like to have this game as polished as the rest of the localizations.

If I understand what you're asking, if you want to see if a given font supports a given Unicode range on Windows, you can use the charmap program which is bundled with Windows. Bring up the font in question and switch the "Group By" drop down to "Unicode Subrange" then scroll down to the language you're interested in. If you see a big bunch of nothing, then that font doesn't support the relevant glyphs. For Japanese, you'll care about both "Japanese Hiragana/Katakana" and "CJK Ideographs". Note that if you try "Group By" with the other settings, it may end up changing which font is being displayed. IIRC, OS X has an analogous program, but I don't recall what it is.

For Windows programs using Far East characters, I generally use the SimSun font by default, because the characters match what I've been brought up with most closely. However, there are regional differences with how people write the same character even in the same country, much less Japanese to Chinese. For example, the grass radical (the top part of this character: ?) in Japanese is generally written with a single continuous horizontal line, but some Chinese (but not all) write it as two lines.