Again, Kirk knows the ship that attacked the Kelvin was Romulan. Something must have happened to give Kirk that knowledge. More than likely it was the logs from the Kelvin or from histories written by people who examined those records. (Perhaps Pike's dissertation) Also, the Romulan language has been known since before the UFP was formed. T'Pol knew the language in the 22nd Century and Hoshi Sato heard it then. The war that followed would also give humans and their allies a great deal of experience with the language. I assume the UT would record the original language as well as give a translation.