ALTDEV ALT OVERSHOT. POTENTIAL CONFLICT THAT WAS ALMOST AN NMAC. LANGUAGE BARRIER PHRASEOLOGY PROB. EVASIVE ACTION LEVEL OFF CLB.

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DURING INITIAL DSCNT AFTER A TRANSATLANTIC XING; PARIS CTL ISSUED A CLRNC TO DSND TO FL260. THE ALT ALERT DEVICE WAS SELECTED TO FL260; I READ BACK FL260; AND THE CAPT WHO WAS FLYING THE ACFT; SET FL260 IN THE FMS. ALL 4 CREW MEMBERS WERE WEARING HEADSETS AT THE TIME. PASSING FL270 A TCASII TA SOUNDED. THE INTRUDER ACFT WAS LOCATED; BOTH ON TCASII DISPLAY AND VISUALLY; AT 11 O'CLOCK OPPOSITE DIRECTION AND TCASII DISPLAYED HIM AT FL260. DSCNT CONTINUED TO FL268; AND AN RA WAS ISSUED BY TCASII TO STOP DSCNT. AT FL268 A CLB BACK TO FL270 WAS INITIATED. AT THIS POINT I QUERIED PARIS CTL AS TO WHAT ALT HE HAD CLRED US TO AND POINTED OUT THE TFC WE HAD. HE SAID WE WERE CLRED TO FL270 AND THE TFC WAS 'NO PROB' BECAUSE HE WAS AT FL260! THE CTLR THEN GAVE US A FREQ CHANGE. I HAVE NO DOUBT THAT IF THE DSCNT WAS NOT STOPPED; THE 2 ACFT WOULD HAVE COLLIDED! THIS IS A TYPICAL READBACK; HEARBACK ERROR. THE LANGUAGE DIFFICULTY AND ACCENT OF FRENCH CTLRS MAY HAVE CONTRIBUTED TO THE PROB. ALL CREW MEMBERS WEARING HEADSETS WILL HELP PREVENT READBACK AND HEARBACK ERRORS. ADDITIONALLY; THE FATIGUE FACTOR AFTER AN ALL NIGHT XING CAN CONTRIBUTE TO ERRORS. WITH ALL OF THESE FACTORS CONSIDERED WE STILL ENCOUNTERED A PROB! SUPPLEMENTAL INFO FROM ACN 273792: 26000 FT WAS PUT IN ALT SELECTOR AND ARMED. I SAID 'WE ARE CLRED TO FL260' AND PUT 26000 FT IN THE FMS. (ALL CREW MEMBERS WERE WEARING HEAD SETS.) WE REMINDED HIM HE HAD CLRED US TO FL260 AND THAT WE READ BACK THE CLRNC TO FL260. IF FL260 WAS NOT THE ALT THE CTLR WANTED US TO DSND TO; HE SHOULD HAVE CORRECTED OUR READBACK. SUPPLEMENTAL INFO FROM ACN 273788: I BELIEVE IF TCASII HAD NOT ISSUED A TA OR RA THAT THE FLT PATHS OF OUR ACFT WOULD HAVE MERGED. PARIS CTL CLRED US TO THE SAME ALT AND IN A CONVERGING VECTOR WITH ANOTHER ACFT. ATC CTLRS SHOULD BE MORE VIGILANT IN HEARING/UNDERSTANDING READBACKS. FRENCH CTLRS ARE VERY DIFFICULT TO UNDERSTAND. ENGLISH SPEAKING AIR CREW MUST MAINTAIN A CAREFUL LISTENING MINDSET.