POTENTIAL CONFLICT BTWN UGA A AND ACR B DUE TO LANGUAGE BARRIER BTWN CTLR AND FLC OF UGA A.

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UGA A; (A C650) WAS INBOUND TO FXE DSNDING FROM FL430 TO FL240 AS CLRED. ACR B WAS HANDED OFF TO US WBOUND. UGA A WAS INSTRUCTED TO AMEND HIS ALT AND TO MAINTAIN FL370. UGA A DID NOT REPLY AND WAS AGAIN INSTRUCTED TO AMEND HIS ALT AND TO MAINTAIN FL370 WHICH HE THEN QUESTIONED. FINALLY ON THE THIRD ATTEMPT HE ACKNOWLEDGED THE AMENDED ALT. ACR B WAS DELAYED IN CHKING ON OUR FREQ AND NOW SEPARATION WAS IN QUESTION. ACR B CHKED ON OUR FREQ AND WAS ASSIGNED FL350 AND A HDG OF 240 DEGS (APPROX A 50 DEG TURN). UGA A WAS ASSIGNED A HDG OF 090 DEGS AND ADVISED TO CLB BACK TO FL390. UGA A ACKNOWLEDGED THE ALT ONLY. UGA A WAS AGAIN ASSIGNED A HDG OF 090 DEGS WHICH HE QUESTIONED AND OBVIOUSLY DID NOT UNDERSTAND. ACR B WAS THEN TURNED 'IMMEDIATELY' FURTHER TO A 200 DEG HDG (A TOTAL OF APPROX 90 DEGS FROM HIS ORIGINAL TRACK). UGA B DID NOT IMMEDIATELY BEGIN A CLB TO FL390 AND WAS AGAIN INSTRUCTED TO TURN TO HDG 090 DEGS AND TO CLB TO FL390. UGA A AGAIN QUESTIONED THE CLRNC. UGA A WAS AGAIN TOLD TO TURN R 'IMMEDIATELY' TO HDG 090 DEGS WHICH HE FINALLY ACKNOWLEDGED; HOWEVER WAS VERY SLOW IN TURNING AND CLBING TO FL390. WITH BOTH ACFT TURNED APPROX 90 DEGS SEPARATION WAS MAINTAINED. THIS IS ANOTHER EXAMPLE OF FOREIGN PLTS WHO DO NOT UNDERSTAND ENGLISH AND DO NOT COMPLY WITH ATC INSTRUCTIONS BECAUSE THEY APPARENTLY DO NOT UNDERSTAND THEM. THESE INCIDENTS OCCUR DAILY; HOWEVER ARE NOT BEING DOCUMENTED BECAUSE SEPARATION IS USUALLY MAINTAINED AT THE EXPENSE OF OTHER ACFT. THESE INCIDENTS GREATLY INCREASE CTLRS WORKLOAD AND JEOPARDIZE AIR SAFETY. THESE INCIDENTS NEED TO BE BETTER DOCUMENTED AND ADDRESSED BEFORE THERE IS ANOTHER INCIDENT SUCH AS THAT WHICH OCCURRED IN NEW YORK WHEN A FOREIGN JET CRASHED.