It looks like Siri will support the Japanese dialect by the time iOS 5.1 has its public release. As the screenshots above show, Siri states Japanese as one of her spoken languages yet a glance at the settings show this isn’t the case.
It’s also rumoured that Mandarin Chinese and Russian will be supported by Siri in the near future too, however I wouldn’t be surprised if Apple included support for many more by iOS 5.1.