AN MD11 CREW; DEPARTING SEOUL; KOREA (RKSS); FLEW THE WRONG SID; SPAWNING QUESTIONS FROM ATC.

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DEPARTING INCHON ARPT; WE FLEW THE WRONG DEP. THE PROB AROSE AFTER NUMEROUS RWY CHANGES AND A MAINT DELAY OF 1 HR. WE HAD 4 DIFFERENT RWY ASSIGNMENTS AND 3 DIFFERENT DEP ASSIGNMENTS. EACH CHANGE REQUIRED A REPROGRAMMING OF THE FMS COMPUTERS. THE FINAL CHANGE WAS A LAST MIN CHANGE WHICH WAS HANDLED BY THE COPLT. IN ACCOMPLISHING THIS LAST MIN CHANGE; WE ACTIVATED THE STORED FLT PLAN WITH THE OTHER RWY. UNFORTUNATELY WE HAD NOT UPDATED IT WITH THE MOST CURRENT SID CHANGE. WE DID NOT CATCH THE OMISSION. ON CLBOUT; ATC CAUGHT OUR MISTAKE WHEN WE WERE ABOUT 5 MI OFF COURSE. THE CONTRIBUTING FACTORS WERE THE NUMEROUS RWY AND SID CHANGES. THE MAINT DELAY WAS ALSO A FACTOR; AS WAS THE LANGUAGE DIFFERENCES. FUTURE PREVENTIVE STEPS WILL BE: 1) MODIFY ONLY ACTIVE FLT PLAN IN COMPUTER. 2) STOP ACFT AND THEN CHK CHANGES. 3) DO A POINT-BY-POINT CHK OF THE CHANGES. 4) XCHK DEP PLATE AGAINST ACTUAL FMS RTE OF FLT. 5) TAKE ONE'S TIME TO ACCOMPLISH ALL TASKS. SUPPLEMENTAL INFO FROM ACN 535840: UPON TAXI OUT; THE DEP RWY WAS CHANGED TO RWY 33L AND THE SECONDARY FLT PLAN WAS ACTIVATED. HOWEVER; NEITHER CREW MEMBER NOTICED THAT THE OSN1A WAS DISPLAYED. DURING CLBOUT; SEVERAL TFC TARGETS WERE CALLED OUT BY DEP CTL AND VERIFIED ON TCASII. UPON ARRIVING AT THE OSN 305 DEG RADIAL; 25 DME; DEP TOLD US THAT WE WERE ON THE ONS1A GND TRACK VERSUS THE TAEA1 TRACK. WE ESTIMATED THAT WE WERE 5-6 MI NE OF THE TAEA1 TRACK.