PBI CTLR DESCRIBED CONFLICT AT 5500 WHEN VFR ACFT FAILED TO FOLLOW CLRNC AND DESCENDED THROUGH IFR ACFT ALT. LANGUAGE BARRIER CITED.

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I WAS WORKING THE 'B' SCOPE POS AT PBI APCH WHEN THE INCIDENT OCCURRED BTWN AN IFR AND VFR ACFT. ALTHOUGH WX WAS NOT A FACTOR; THERE WERE NUMEROUS CELLS AFFECTING FLT THROUGHOUT THE AIRSPACE. WHERE THE INCIDENT OCCURRED; IT WAS NOT A FACTOR. ACFT Y (LJ30) ON AN IFR FLT PLAN; HAD DEPARTED SUA ARPT ENRTE TO BCT ARPT AT 5000 FT MSL. THE PLT HAD INQUIRED ABOUT A VFR AHEAD OF HIM AT 5500 FT FROM HIS TCAS. I ISSUED THE TFC WHICH; AT THE TIME; WAS 12-15 MI AHEAD. I CALLED ACFT X (C172) AND ISSUED THE LJ30 TFC. DURING THE XMISSION; I SPECIFICALLY INSTRUCTED ACFT X NOT TO DSND UNTIL ADVISED AS THE LEARJET WOULD BE OVERTAKING HIM FROM BELOW. I TURNED MY ATTN TO OTHER ACFT IN MY SECTOR. I LOOKED BACK TO THESE 2 ACFT; WHICH WERE JUST NOW PASSING EACH OTHER AND OBSERVED ACFT X'S MODE C INDICATING 3500 FT. I ASKED THE PLT TO VERIFY THE ALT AND HE VERIFIED THE ALT; 3500 FT. I ASKED THE PLT SOMETHING CLOSE TO THE FOLLOWING: 'DID I NOT TELL YOU TO REMAIN AT 5500 FT UNTIL ADVISED FOR TFC?' TO THIS; THE PLT RESPONDED; 'I DSND TO 3500 FT.' I ASKED THE SAME QUESTION AGAIN; THIS TIME REMINDING HIM ABOUT HIS TFC; AND ONCE AGAIN THE RESPONSE WAS THE SAME; 'I DSND TO 3500 FT.' THE PLT OF ACFT Y MADE NO COMMENT ON THE FREQ. THE PLT THIS DAY SPOKE VERY BAD ENGLISH IN A FOREIGN ACCENT. MOST TIMES; IN RESPONSE TO SUCH AN INCIDENT; THESE FOREIGN PLTS RESPOND WITH THE PHRASE; 'I AM SORRY.' IN THIS CASE IT SOUNDED MUCH MORE LIKE 'HE-DID-WHAT-HE-DID AND I JUST HAD TO ACCEPT THAT HE-DID-IT.' THIS IS VERY COMMON IN SOUTH FLORIDA WITH FOREIGN PLTS. I WOULD LIKE TO ADD MORE BUT I WOULD ONLY BE GIVING MY OPINION WHICH I WOULD RATHER RESERVE FOR ANOTHER TIME.