The flightcrew inadvertently flew into an area of clear air turbulence. Weather forecasts indicated that there was not to be turbulence in the area where it was encountered. The ability to forecast clear air turbulence over the ocean is limited. In addition, the Japanese-speaking flight attendant did not fully translate one of the captain's instructions to fasten seat belts into Japanese, although such an announcement was made and translated several times prior to the turbulence encounter. Most of the passengers spoke only Japanese.