When iOS 5.1 Beta 3 went online, I immediately went to work, grabbed the delta packages (since beta 3 is OTA), and unpacked them. (At least the iPhone 4 and 4S packages.)
There’s the usual updates on this and that, but in the iPhone 4S package in the “added” folder (that’s new files added to iOS with this delta package) in /System/Library/PrivateFrameworks/AssistantUI.framework there was a new folder, called ja.lproj. That’s Japanese strings! I opened it… et voilà!
So there is the absolute path as proof, the files, and two example plists.
I don’t know Japanese (just German, English and a little French), but one of the strings in Calendar.plist translates to “Make a schedule change” (according to Google Translate.)
As a bonus, here’s all the strings: grab the zip file.
I can’t tell a 100 percent sure if Siri is coming to Japan with iOS 5.1, but as of today, it’s apparently not available there, and this looks like Japanese 4S customers will be able to talk to their phones soon :-)