Sorry for the late reply, I had to get on lunch to really make a reply to this one. So basically it's still piracy to download the Japanese version of the same game you already own. You legally own the US version, but that doesn't mean you own versions from other regions. Unless it were the case where the game is the same across all regions. Otherwise you only own that regional version. Now I am not 100% sure on this one since copyright laws are rather poorly equip to deal an issue like this. Not to mention country differences in copyright laws. But I am going to with the stance of, you legally only own the copy of the game you bought. Otherwise the titles are treated as two separate titlekeys. But like I said, this is something I am not 100% sure on and will have to do further research.