Edit: Wow, I didn't read this one well. Try creating the ISO using Wiimm's ISO Tools.
wit copy x:\game.iso x:\game\
will copy every file from the game into a folder called game. Then find the file you want to replace in the folder and replace it. Make sure the filename is identical. Then use WIT again to make the new ISO.
wit copy x:\game\ x:\game-es.iso
Original post; may still be useful if your game freezes or something:
Try editing with a different hex editor like HxD or MadEdit. Take extra care not to add text before the start of or after the end of where the text is in English. Check the header of the file (at the very beginning) and Google to see if it's a known file format. If it is you may be able to decode and re-encode the script into plain and editable text.
Perhaps the game is trying to load Spanish characters from a font that doesn't have any.