Today, the XNA community gaming platform (CGP) team released a Japanese version of the XNA Game Studio 2.0 documentation. You can download the installer for this documentation from this location. It can be installed on any system that has Visual C# 2005 Express Edition or Visual Studio 2005 Standard Edition or higher installed. If you already have XNA Game Studio 2.0 installed on your system, you can install the Japanese documentation side-by-side and access both the English and the Japanese documentation in the help viewer.
If you read Japanese and would like to view the existing XNA Game Studio 2.0 help documentation in Japanese instead of (or in addition to) English, then I encourage you to download the XNA Game Studio 2.0 Japanese documentation installer and give it a try.