BE90 PLT AND FOREIGN CTLR HAD LANGUAGE BARRIER; READBACK HEARBACK PROBS IN A FOREIGN LOCATION.

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OUR COMPANY WORKS IN CONJUNCTION WITH THE USDA ON THE INSECT PEST ERADICATION PROGRAM HERE IN ZZZ; FO. WE RELEASE STERILE FLIES OVER THE COUNTRY IN GRID OR BLOCK PATTERNS. IN THE PAST; WHEN THE WX NECESSITATED AN IFR FLT PLAN; WE WERE CLRED TO OUR DISPERSAL REGION AT AN ALT WITH FURTHER CLRNC BACK TO LOCATION. A CHANGE IN ATC PROC REQUESTED A BLOCK FROM BTWN 2 RADIALS AND 2 DME SETTINGS FROM ZZZ VOR WHILE AIRBORNE. I GAVE THE 2 RADIALS BTWN WHICH I INTENDED TO FLY AND THEY CLRED ME TO FLY BTWN THOSE RADIALS AT 80-120 DME. I ACCEPTED. LATER WHEN I REALIZED THAT THE DME DISTANCES WOULD NOT ALLOW FULL COVERAGE OF MY REGION; I REQUESTED AN AMENDMENT FOR A BIGGER BLOCK AND WAS GIVEN AN EXTENSION. TOWARDS THE END OF MY BLOCK I WAS UNSURE OF AMENDED CLRNC COVERAGE NEEDED TO OVERFLY SOME NEARBY ISLANDS; SO STATED MY INTENTIONS OF FLYING TO THEM. IT WAS ACKNOWLEDGED AND I WAS TOLD TO MAINTAIN MY ALT. AFTER LEAVING THE ORIGINAL BLOCK; ATC INFORMED ME THAT I WAS NOT CLRED TO DO SO AND I HAD VIOLATED MY ORIGINAL CLRNC. I REQUESTED FURTHER CLRNC WHICH WAS GIVEN AND COMPLETED THE FLT. CONTRIBUTING FACTORS INCLUDE: 1) SPANISH/ENGLISH LANGUAGE BARRIERS. 2) MY ASSUMPTION OF BEING FULLY CLRED TO FLY OUTSIDE OF AN ASSIGNED BLOCK. 3) CHANGE IN ATC PROC LEADING TO AMBIGUITY. 4) HVY TFC FLOW ON ATC FREQ. CORRECTIVE ACTIONS: OUR BASE IS PLANNING A VISIT TO ATC TO DISCUSS EFFECTIVE PROCS AND ANY CONCERNS.