You need a program I have in order to insert it back.
You will only go one way:
From human readable text (in the Google Sheet)
to the same text, but in a format the game can read.
There's no way any game would be able to read human readable text.
There's 10,000 lines in the Google Sheet. Feel free to do as much or as little as you like. Be sure to release a patch when you're finished.
Inserter:
http://pastebin.com/TvEGwD2e
1. Get your input file data_msg_txt.tsv.
1a. Go to Google Sheet
1b. Push on the menu bar TSV: Export current sheet to tsv.
1c. Give permission. You only have to do this the first time.
1d. The file is in your Google Drive. You can download it or do whatever.
2. Run program
2a. Download the program from pastebin (only 1st time).
2b. Put data_msg_txt.tsv in the folder with the program.
2c. Run program using Python 2.7
2d. You wind up with data_msg_txt.dat which can be put back into the game.
3. Build the ISO with the new data_msg_txt.dat file. I think you already know how to do this.