If you have JPN Visual Studio 2008 installed, you can install the Runtime which has Japanese resources inside and SLTools below and create the Silverlight apps targeting version 3.

Runtime:
http://go.microsoft.com/fwlink/?linkid=143433

Silverlight 3 Tools for Visual Studio 2008 SP1 (Japanese):
http://www.microsoft.com/downloads/details.aspx?displaylang=ja&FamilyID=11dc7151-dbd6-4e39-878f-5081863cbb5d

SDK (Japanese):
http://www.microsoft.com/downloads/details.aspx?displaylang=ja&FamilyID=d09b6ecf-9a45-4d99-b752-2a330a937bc4

Documentation (English):
 http://msdn.microsoft.com/en-us/library/cc838158(vs.96).aspx.

Blend 3 Preview (English):
http://www.microsoft.com/expression/blendpreview