If you have spanish then the disks you installed with are for spanish. I could be wrong here but I think the only way to solve this is to use english disks. The langauge is set at the time of install & I am not sure if you can use english on disks for spanish.
I know that the english version has two files called useenglishbig & it is these two files that make it english but I do not know the names of the files the spanish version uses it might be usespanishbig. But this is only a guess. 8)