Glib is correct, `Greek Small Letter Alpha with Psili` is not a character in ISO-8859-1, see https://en.wikipedia.org/wiki/ISO/IEC_8859-1.
ISO-8859-1 only encodes 256 characters of the million or so that Unicode does and Greek characters are not included, so it can't be converted. If the file contains a Greek character then your claim that you only need Spanish is false (at least as far as that file goes).