Date: 201810
Local time of day: 0001-0600
Location: RJJJ.ARTCC, FO
Flight conditions: VMC
Weather elements / visibility: Turbulence
Aircraft: Widebody; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
ATC / advisory: Center RJJJ
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Communication Breakdown
Second reporter function: Captain
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Human Factors; Weather
Primary problem: Ambiguous

Narrative: At approx XX:16Z company sent us a SIGMET message outlining an area of forecast severe turbulence. Area was plotted and lay north and west of our route of flight; moving northeast; which would parallel initially; then diverge from our intended route. At approx XX:43Z; while on course between SYOYU and KAGIS we were cleared DCT AVBET. We were in stabilized cruise at FL310 and approx .82Mach. At approx XY:25Z another company aircraft (Aircraft Y) tried to warn RJJJ control of an encounter with severe turbulence; and to not allow following aircraft to enter that area. Due to severely limited English capability; the controller could not understand the company aircraft's request. We also tried to ascertain the location of the severe turbulence encounter but RJJJ control was unable to understand our request for the same severe language barrier issues. Finally; after 2 to 3 minutes trying; we elected to turn right; away from the area of forecast severe turbulence and away from an aircraft we assumed to be [Aircraft Y] depicted on TCAS. We requested 20 NM right of track then turned about 30 degrees right. We then asked for 30 NM right deviation. At approx XY28Z we encountered severe turbulence which caused us to climb 550 feet in about 2 seconds; and experience both stick shaker and airframe buffet. We immediately notified RJJJ control of the encounter and altitude deviation. At the time; we were in what appeared to be clear air with no radar returns at our altitude. [Aircraft Y] confirmed their encounter also showed nothing at flight level using EFVS and no radar returns at their flight level. Autopilot and auto throttles remained engaged throughout the encounter. PF (Pilot Flying) used vertical speed to return the aircraft to FL310. Nothing beyond moderate turbulence was encountered for the remainder of the flight. Remainder of the flight; descent; approach and landing were uneventful. Inadvertent encounter with one area of severe turbulence. The inability of RJJJ control to understand the problem and suggest alternate routing contributed to the encounter.Turbulence; often moderate; is routinely encountered along this route of flight. Perhaps reviewing routing options and choosing one that avoids this area would be beneficial.


Second narrative: [Report narrative contained no additional information.]


Synopsis: Air Carrier flight crew reported communications difficulty with foreign ATC due to language barrier.