RWY INCURSION.

Narrative

AFTER LNDG ON RWY 28 AT GDL; TWR ISSUED INSTRUCTIONS FOR CLRING THE RWY AND TAXI WHICH SOUNDED TO OUR CREW LIKE 'USE RWY 20.' THE COPLT ACKNOWLEDGED WITH 'ROGER.' THE CAPT EXITED RWY 28 AT RWY 20 (R TURN TO 020 DEGS) AND SHORTLY THEREAFTER; TWR TOLD US TO IMMEDIATELY EXIT THE RWY 20. IT WAS THEN THE COPLT NOTICED A LIGHT ACFT EXECUTING WHAT APPEARED TO BE A GAR FROM AN APCH TO RWY 20. APPARENTLY; WE HAD BEEN INSTRUCTED 'NOT' TO USE RWY 20 FOR OUR TURNOFF. THROUGHOUT OUR APCH AND LNDG; TWR SPOKE EXCLUSIVELY IN SPANISH EXCEPT WHEN SPEAKING TO US. THIS PREVENTED ANY SITUATIONAL AWARENESS ON OUR PART. WHEN ENGLISH WAS SPOKEN TO US; IT WAS VERY DIFFICULT TO UNDERSTAND AND WITH A HVY ACCENT. CONTRIBUTING TO THE CONFUSION WAS THE FACT THAT ON OUR 2 PREVIOUS TRIPS TO GDL; WE HAD BEEN DIRECTED TO TAXI VIA RWY 20/02. LESSONS LEARNED: 1) DON'T USE A RWY AS A TURNOFF IF COMS ARE DIFFICULT OR AMBIGUOUS. 2) NEVER SIMPLY 'ROGER' A CALL INVOLVING ENTERING OR EXITING A RWY; BUT RATHER; READ BACK THE ENTIRE CLRNC.