WIDEBODY CARGO TRANSPORT FLC RECEIVE AND ACKNOWLEDGE VTBD CTLR CLRNC TO CROSS RWY 21R AND INTO POSITION ON RWY 21L WITH TFC ON FINAL; WHO QUESTIONS THEIR XING. CHANGE OF CTLR ADVISES THAT CLRNC WAS 'AFTER' LNDG TFC.

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WE WERE CLRED TO TAXI TO RWY 21R. AS WE TAXIED; WE WERE ADVISED TO EXPECT RWY 21L. I STARTED CHANGING RWY INFO IN THE FMC AND CHECKING TKOF DATA FOR THE NEW RWY. WE WERE SWITCHED TO TWR FREQ. TWR SAID SOMETHING ABOUT ACFT ON FINAL WHICH WE DIDN'T FULLY UNDERSTAND AND THEN SAID CROSS RWY 21R AND LINEUP RWY 21L. THERE WAS AN ACFT ON FINAL. IT WAS DARK AND WE COULD ONLY SEE THE LNDG LIGHT. THE LIGHT APPEARED FAR ENOUGH OUT THAT THERE WAS NO CONFLICT. IT IS OFTEN IMPOSSIBLE TO TELL AT NIGHT IF AN ACFT IS 3 MILES OR 10 MILES AWAY. I ASSUMED THE CTLR HAD TOLD US THE ACFT WAS 5-10 MILES OUT AND CLRED US TO CROSS; SO I READ BACK 'CLRED TO CROSS RWY 21R AND LINE UP RWY 21L.' WE HAD NOT COME TO A STOP AND THE TWR DID NOT CHALLENGE OUR READ BACK; SO THE CAPT CONTINUED ACROSS RWY 21R. AS WE WERE ON THE RWY LOOKING AT THE LIGHT ON FINAL; I FIRST BEGAN TO SENSE RELATIVE MOTION AND SAID TO THE CAPT 'THAT IS CLOSE.' AT THE SAME TIME; THE ACFT ON FINAL TRANSMITTED. 'WE HAVE AN ACFT XING THE RWY.' WE CLRED THE RWY WHEN; IN MY JUDGEMENT; THE ACFT WAS 1/2 -1 NM OUT. THE TWR (A DIFFERENT CTLR) CALLED AND SAID 'YOU WERE TO CROSS THE RWY BEHIND THE ACFT ON FINAL.' THIS WAS A 'CONDITIONAL CLRNC.' ALLOWED BY ICAO RULES. I REPLIED 'THAT IS NOT WHAT WE HEARD AND NOT WHAT I SAID' (MEANING NOT WHAT I READ BACK). WE LINED UP ON RWY 21L AND WERE CLRED TO TKOF. I FEEL THE ROOT CAUSE/ PROB HERE IS THE USE OF CONDITIONAL CLRNCES. I HAVE ALWAYS THOUGHT THEY WERE DANGEROUS BECAUSE; JUST AS HAPPENED HERE; IF YOU DON'T HEAR THE FIRST PART (AFTER LNDG TFC; ETC.) CORRECTLY; YOU CAN PULL OUT IN FRONT OF SOMEONE ON FINAL. THE ONLY SAFE WAY TO GO IS FOR TWR NEVER TO ISSUE CLRNC ONTO A RWY UNTIL LNDG TFC HAS CROSSED THE THRESHOLD. A CONTRIBUTING FACTOR WAS THAT WE ASSUMED THAT SINCE OUR READBACK WAS UNCHALLENGED; IT WAS SAFE TO CROSS EVEN THOUGH THE CTLR WAS HARD TO UNDERSTAND. HOWEVER; IN THIS SIT; BOTH OF US THOUGHT THE ACFT ON FINAL WAS FAR ENOUGH OUT AND THAT WE HAD CLRNC FROM TWR. USUALLY; ONE GUY HAS DOUBTS IF THINGS AREN'T RIGHT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED HE FLIES INTO VTBD 3 TO 4 TIMES A MONTH; STATING NEITHER PLT HAD A QUESTION ABOUT THE CLRNC. ONLY PASSING ACROSS THE RWY DID HE AND THE CAPT MORE CLEARLY RECOGNIZE THAT ANOTHER ACFT WAS ON SHORT FINAL FOR RWY 21R AND NOT FURTHER OUT ON FINAL AS EXPECTED. THE RPTR ADVISED THAT ONLY WHEN A QUESTION OF PLT CLRNC COMPLIANCE DOES ANOTHER CTLR WITH BETTER ENGLISH PROFICIENCY COME ON THE FREQ. THE RPTR EXPRESSED GENERAL CONCERN ABOUT CTLR ENGLISH PROFICIENCY THROUGHOUT THE ASIA SUBCONTINENT. THE RPTR STATED THAT ALMOST ALL CLRNC READBACKS HAVE A PLT QUESTION IN THEIR MIND AS TO 'DID WE GET THAT RIGHT;' OR 'DID THEY UNDERSTAND US?'