A disclaimer on the Xbox website that mentioned an "Xbox One-supported Xbox Live country" lead folks on Twitter to message Xbox support in an effort to find out exactly what that means. Their answers were less than positive. One user asked about traveling outside of an Xbox One-supported region with his console and he was told that he could play once he returned home. The rep then said, "I travel with an Xbox 360 for road gaming."
Another user asked about importing games, and they were told to wait until the Xbox One becomes available in their region.
Finally, someone asked directly if they could play from an unsupported country if they created an Xbox Live account based in another, supported country. The response they received was that they wouldn't get the "full experience," and, more to the point, it "may not work."
If you don't live in one of the 21 supported launch regions (and Japan is not one of those regions), you may not be able to play Xbox One at all. The Verge attempted to get a straight answer from Microsoft, but as with the earlier used game debacle, the company's responses are anything but straightforward.