CLRNC MISUNDERSTANDING RESULTING IN A RWY INCURSION IN BANGKOK; THAILAND.

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WE WERE CLRED TO TAXI TO RWY 21R; BUT AS WE TAXIED; WE WERE TOLD TO EXPECT RWY 21L. THE CTLR WAS DIFFICULT TO UNDERSTAND. AS WE APCHED THE HOLDING POINT FOR RWY 21R; WE WERE CLRED 'CROSS RWY 21R AND LINE UP RWY 21L;' WHICH THE FO READ BACK. THERE WAS AN ACFT ON FINAL; BUT I BELIEVED SPACING WAS ADEQUATE AS WE HAD NOT STOPPED AT RWY 21R. AS WE CROSSED THE RWY; THE ACFT ON FINAL XMITTED; 'WE HAVE AN ACFT XING THE RWY;' AND THE TWR CALLED US AND SAID; 'YOU WERE TO CROSS BEHIND THE ACFT ON FINAL.' THE FO REPLIED; 'THAT IS NOT WHAT WE HEARD OR WHAT WE READ BACK.' WE LINED UP ON RWY 21L; AND WERE CLRED TO TKOF. THE CTLR; WHO HAD ISSUED THE CLRNC TO CROSS THE RWY; HAD BEEN HARD TO UNDERSTAND; AND WE BOTH ASSUMED THAT THE FO'S READBACK; WHEN UNCHALLENGED; WAS ASSURANCE WE HAD UNDERSTOOD THE CLRNC CORRECTLY. A MORE PRUDENT COURSE OF ACTION WOULD HAVE BEEN TO STOP THE ACFT AND QUERY THE CTLR; ESPECIALLY GIVEN THE CONFUSION POSSIBLE DUE TO THE USE OF CONDITIONAL CLRNCS IN ICAO OPS (IE; 'BEHIND THE LNDG ACFT; CROSS RWY 21R AND LINE UP RWY 21L').