I haven't messed with it in forever but the general process goes like this:
UMD Gen, Extract Install CPK.
CPK Tools, Extract data with the Install CPK.
Use the Madohack "decodeBTX", "decodeCSB", and "decodePAC" programs to extract text from various files. I can't recall which files contain story data, I believe it's in the 00_data/GameData.pac but I'm not 100% certain anymore. It should be mentioned earlier in the thread.
After editing the text using a text editor use the "encode" programs to create a re-built version of your file. Then put the file in a folder with the other translated files (Note: Keep 2 folder, 1 original and 1 translated. Both should be identical except the translated has the new files replacing the old ones) and use the CPK tools to create a new Install CPK. Make sure to leave force compression off. Then replace the original Install CPK using UMD Gen (Drag and drop method) and save it as a new ISO. DO NOT OVERWRITE YOUR ORIGINAL ISO. Congratulations, you've not made an edited version of Danball Senki BOOST.