ACFT TAKES ACTIVE RWY IN FRONT OF LNDG ACR 2 MI OUT REQUIRING THE LNDG ACR TO GAR.

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ON APCH TO MEXICO CITY WE WERE CLRED FOR THE ILS RWY 5R. WHEN WE FIRST CHKED IN WITH TWR AT 2000 FT AGL; WE WERE TOLD TO CONTINUE. AT 1000 FT AGL; WE WERE CLRED TO LAND. AT 700 FT AGL WE NOTICED A B-737 TAXIING ONTO THE EXTENDED THRESHOLD FOR RWY 05R. THE B-737 APPEARED TO BE TAKING OFF. WE WERE ABOUT TO CALL MISSED APCH WHEN THE TWR TOLD US TO GAR. AT 400 FT AGL; WE EXECUTED A GAR AND WERE VECTORED BACK AROUND FOR ANOTHER ILS TO RWY 05R. WE SHOT THE APCH AND MADE A NORMAL LNDG. FACTORS: COM WERE VERY POOR AS A RESULT OF SPANISH ACCENTS ON ENGLISH WORDS. WE DID NOT KNOW IF THE OTHER ACFT WAS CLRED INTO POS OR CLRED FOR TKOF BECAUSE THE OTHER AIRCREW WAS SPANISH SPEAKING AND THE TWR AND THE B-737 WERE USING SPANISH. HAD THEY BEEN USING ENGLISH WE WOULD HAVE HEARD ANY CONFLICTING CLRNC GIVEN TO THE B-737. HUMAN FACTORS WERE POOR CTLING BY MEXICO CITY TWR AND/OR POOR AIRMANSHIP BY THE B-737 CREW. THE TWR CTLR GAVE US VERY POOR GAR INSTRUCTIONS. HE SAID; 'GAR TURN L.' NO HDG ALT OR NAV INSTRUCTIONS WERE GIVEN.