Can I replace the system font just for Japanese text? I discovered today that the system font on my rooted Galaxy S2 phone has some errors in the way it renders some Japanese characters.

I want to change the font to something I'm sure is more reliable, but I'm unsure that I can change just the Japanese, and impact all fonts.

I have looked into font changing apps, but I'm a little nervous as there are a lot of comments suggesting the potential to brick the phone. So I want to proceed carefully.

I don't really know how font handling works, especially with an Android device, so is it possible for me to change the font for just the Japanese text?
The way font handling works is your android device has files in the system/fonts folder that control the font for certain aspects of the android system. One of them is the clock, the text, etc. 

In order to change these you need to rename your font file (let's say its helvetica.ttf) to android-clock.ttf to change the font of the clock. Do this for all the files in the system/font folder to change all the font on your phone.

Now, the Japanese characters... I'm not sure if the characters are handled differently than the english characters. I assume they're not, and this is why when you change the font they look strange. It's probably because the font file doesn't have the ability to change how these characters work, and is not sure how to deal with them. You may have to find a font that is compatible with both english/japanese characters.