I think you can just edit that in raw jus using a hex editor since its clearly not compressed.
Cant see the file yet, but i've translated part 1 of that game just by hex editor
*edit
took a quick look and tried making a .bms script using my noob skills so don't expect much this just rips the main files from the .cat, but individual files also contains its own table/pointers for scripts/text (I think)