RWY INCURSION BY MD80 AT MMMX CLRED TO RWY 5L INSTRUCTED TO FOLLOW AIRBUS. AIRBUS CROSSES RWY 5L FOR RWY 5R AND MD80 CREW ALSO CROSS.

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ON TAXI OUT FROM GATE ATC ASSIGNED 'TAXI TXWY B TO RWY 5L AT TXWY B2. FOLLOW ACR Y AHEAD.' APCHING TXWY B2 ATC ADVISED 'SWITCH TO TWR 118.1' ACR Y TURNS ONTO RWY 5L AT TXWY B2 AND BEGINS BACK TAXIING TO THE DEP END. OUR FLT TURNS AT TXWY B2 AND ENTERS THE RWY AND BEGINS BACK TAXIING BEHIND ACR Y AHEAD. ACR Y TURNS L AND ENTERS TXWY A1 SHORT OF RWY 5R AND OUR FLT FOLLOWS; ENTERING TXWY A1 BEHIND ACR Y; SHORT OF RWY 5R. TWR CLRS ACR Y FOR TKOF ON RWY 5R AND THEN ASKS US OUR POS; WHEREIN WE ARE ADVISED WE WERE NOT CLRED TO BACK TAXI ON RWY 5L. CONTRIBUTING FACTORS TO THIS INCIDENT WERE THE LACK OF STANDARD PHRASEOLOGY; FOREIGN LANGUAGE DIFFICULTY; TXWY B CLOSED FOR CONSTRUCTION PAST TXWY B2; AND THE CAPT HAVING A 'FOLLOW THE HERD' MENTALITY. I SHOULD HAVE HELD SHORT OF RWY 5L AT TXWY B2 AND WAITED TO BE CLRED TO BACK TAXI AS THE ATC WAS DOING WITH OTHER TFC IN SPANISH. SUPPLEMENTAL INFO FROM ACN 595495: DURING TAXI ON TXWY B AT MEX; GND CTL INSTRUCTED US TO TAXI TO RWY 5L VIA TXWY B2; TO FOLLOW THE ACR Y AIRBUS IN FRONT OF US; AND TO SWITCH TO TWR ON 118.1. WE COULD SEE WE WOULD HAVE TO BACK TAXI ON RWY 5L IN ORDER TO BYPASS THE CONSTRUCTION ON TXWY B BTWN TXWY B2 AND THE DEP END OF RWY 5L. WE SAW THE ACR Y AIRBUS BACK TAXIING ON RWY 5L SO WE CONTINUED TO FOLLOW IT. WE PULLED UP BEHIND IT ON TXWY A1 HOLDING SHORT OF RWY 5R. BEFORE ENTERING RWY 5L WE CHKED THAT THE FINAL APCH WAS CLR OF TFC. AFTER THE ACR Y FLT WAS CLRED FOR TKOF TWR ASKED OF OUR POS. I TOLD THEM WE WERE HOLDING SHORT OF RWY 5R ON TXWY A1. THE TWR CTLR INFORMED US THAT WE SHOULD HAVE RECEIVED A CLRNC AT TXWY B2 TO BACK TAXI ON RWY 5L. EVEN THOUGH WE ENSURED RWY 5L WAS CLR OF TFC AND WE SAFELY TAXIED TO TXWY A1; WE SHOULD HAVE FALLEN BACK ON THE BASICS PROVIDED IN AIM TO HOLD SHORT OF RWY 5L AT TXWY B2 AND THEN INQUIRED ABOUT FURTHER TAXI INSTRUCTIONS TO THE TKOF RWY.