ENRTE ALT CHANGE OVERSHOT WHEN FO; PF; HEARS CLRNC TO FL300 VERSUS ATC ALLEGEDLY ASSIGNED FL370 80 MI S OF MYDIA INTXN; FO.

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FLT FROM SAN JOSE; COSTA RICA; WAS 80 MI S OF MYDIA INTXN ON HIGH ALT RTE UB881; BOUND FOR ATLANTA; GA. THE FO WAS FLYING THE ACFT ON AUTOPLT AT FL390. THE CAPT WAS CALLING ZHU 10 MINS PRIOR TO ENTERING US AIRSPACE. WHILE THE CAPT WAS ON RADIO #2; MERDIA CTLR ON FREQ 128.2 CLRED THE FLT TO FL300. THE FO ACKNOWLEDGED THE ALT CHANGE AND STARTED THE ACFT DOWN. WHEN THE CAPT COMPLETED HIS CALL; THE FO RELATED THIS INFO TO THE CAPT. PASSING FL334; MERDIA CTR ASKED OUR ALT AND INSTRUCTED US TO RETURN TO FL370. THEY TOLD US WE WERE NOT CLRED TO FL300. WE CLBED TO FL370 WITH NO FURTHER CONVERSATION OR DIFFICULTY. FOREIGN ACCENTS WERE CERTAINLY A CONTRIBUTING FACTOR. VOICELESS COM WOULD HAVE BEEN A GREAT HELP. SUPPLEMENTAL INFO FROM ACN 582591: WE WERE TOLD TO EXPECT FL300 IN A FEW MINS. THE FO; WHO WAS FLYING; WAS THE ONLY ONE ON FREQ. THE CTLR SAID DSND AND MAINTAIN FL300. I HEARD FL300; I READ BACK FL300. WE DO NOT BELIEVE SEPARATION WAS LOST. LANGUAGE WAS DEFINITELY A CONTRIBUTING FACTOR TO THIS INCIDENT. NORMALLY BOTH PLTS ARE LISTENING TO XMISSIONS TO AVOID THIS TYPE OF CONFUSION BUT THE TIMING WAS RIGHT WHERE THE CAPT HAD TO CALL AHEAD. I AM CERTAIN I HEARD FL300 AND I KNOW I READ BACK FL300.