FLC OF A B727-200 ACCEPTED ANOTHER ACFT'S CLRNC AND CLBED TO THAT ALT. ARTCC RADAR CTLR INTERVENED AND ADVISED THAT THEY HAD NOT BEEN CLRED TO THE ALT TO WHICH THEY WERE AT AND THEN CLRED THEM TO A HIGHER ALT.

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DURING A FLT FROM LEON; MEXICO; TO LAREDO; TX. WE EITHER MISHEARD A MEXICAN CTLR OR CTLR ISSUED A CLB CLRNC WHICH SHE LATER QUESTIONED. I WAS FLYING THE PLANE AND CAPT WAS HANDLING THE RADIOS WITH MYSELF AND THE FE MONITORING. OUR FLT WAS BEING STEP-CLBED TO OUR FLT PLANNED ALT AND WE WERE CURRENTLY AT FL210. THE CTLR WORKING ON OUR PARTICULAR SECTOR WAS SWITCHING BACK AND FORTH BTWN ENGLISH WITH A HVY ACCENT AND SPANISH. AT ONE POINT; SHE MENTIONED OUR CALL SIGN AND SAID SOMETHING UNINTELLIGIBLE; FOLLOWED BY 'MAINTAIN TWO-TWO-ZERO.' WE INTERPED THIS TO MEAN CLB TO FL220. CAPT READ BACK TO THE CTLR THE NEW CLRNC; 'CLB TO FL220 ACR X.' I CONFIRMED CLRNC WITH CAPT ON THE ALT ALERTER AND COMMENCED A CLB TO FL220. SEVERAL MINS LATER SHE QUERIED US AS TO OUR ALT. WE INFORMED HER WE WERE AT FL220 PER HER CLRNC. SHE TOLD US SHE HAD NOT CLRED US TO THAT ALT. SHE THEN TOLD US TO CLB TO ANOTHER ALT. WE CONFIRMED AND EXECUTED THIS CLRNC. THE IMPORTANT INFO LEADING TO THIS ALTDEV WAS THE CONFUSION RESULTING FROM CTLR SWITCHING LANGUAGES IN HER XMISSIONS AND THE FACT SHE DEFINITELY SAID 'MAINTAIN 220' WITH OUR CALL SIGN. THE 'MAINTAIN 220' WAS AN ALT ASSIGNMENT AND NOT A HDG ASSIGNMENT BECAUSE WE WERE ON A NE HDG FOR THE VOR.