ALT EXCURSION CAUSED IN GOOD PART BY LANGUAGE DIFFERENCES. THIS CREATED A TFC CONFLICT WHERE ANOTHER AIRLINER TOOK EVASIVE ACTION.

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FLYING FROM SAN PEDRO DE ZULA (HONDURAS) TO SAN JOSE (COSTA RICA) WE WERE CLR FROM EL COCO APCH TO DSND TO FL190 WITH A QNH 29.96 AND IN RADAR CONTACT. WHEN WE RPTED LEVELOFF AT FL190; ATC RPTS THAT WE HAVE TFC CLBING TO FL180. IN THAT EVENT WE WERE FLYING IMC; IN A TSTM WITH RAIN AND SEVERE TURB. I WAS FLYING THE ACFT AND THE FO ON THE RADIO UNDERSTOOD THAT WE WERE CLR TO FL180. SO DID I. THE FE ALSO HEARD THAT THE SAME WAY; BECAUSE WE WERE 3-MAN CREW. SO WE RPTED AND DSNDED FROM FL190 TO FL180 AND WE DIDN'T HAVE A READBACK FROM ATC. AS SOON AS WE LEVELED OFF AT FL180 WE RPTED TO ATC; AND HE TOLD US THAT WE WERE NOT CLRED TO THAT ALT; SO CLBED IMMEDIATELY TO FL190. THE TFC; THAT WAS AN ACR AIRLINE; RPTED BY HIS TCASII THAT HE HAS TFC; THE SAME ALT ; AT 6 MI FROM HIM. SO; HE RPTED AN EVASIVE ACTION; MAKING A R TURN. I THINK THIS WAS A MISUNDERSTANDING BTWN ATC AND US BECAUSE WHEN WE RPTED LEAVING FL190 TO FL180 WE DID NOT HAVE A READBACK OF THE RPT. AS SOON AS WE WERE ON THE GND WE WENT TO THE TFC CTL OFFICE TO HEAR THE RECORDING FROM ATC AND TALKING TO THE CTLR; HE SAID THAT WHEN WE RPTED LEAVING FL190 TO FL180 HE THOUGHT THAT WE WERE READING BACK THE TFC INFO; CLBING TO FL180. FLYING A LOT OF TIME IN CENTRAL AND SOUTH AMERICA; I EXPERIENCE THAT ATC CTLRS DON'T HAVE FLUENT TALKING AND UNDERSTANDING OF THE ENGLISH LANGUAGE; AS THE WAY HAS TO BE CONSIDERING THAT ENGLISH IS THE UNIVERSAL AND INTL LANGUAGE IN AVIATION.