Don't change the extension of the original file.
Instead, use the 'Open With...' option in the menu and use notepad (I use Cygnus since its a hex editor)
Alternatively use some tools to extract the msgX.dat (where X = Stage #) from th07e.dat for direct editing.
msg1.dat:
It looks more editable.
Then pack it using datpacker.
However the translations.pickle is more complete since it has all the stage translations in it. However, they're jumbled and you have to find the specific lines to translate using ctrl-F:
translation.PICKLE using a notepad:
Which means you should know what you want to edit. Pickle is an archive used by the Python language, so it's encoded and a headache to edit.
Some Tests editing msg1.dat
Editing the Boss Names in msg1.dat
I was able to edit the spellcard.eng in the patcher then patched it without problems:
It follows the same rule as editing the msg.dat that it needs the same number of characters in each line (including spaces)
I don't know how to edit the bomb names yet.
Chances are what you did was:
Edit spellcard.eng -> patch -> patch failed but since it was done without using the normal procedure there was no error log (actually there was but it displayed too fast to be noticed) -> incomplete .dat file was created -> corrupt .dat file was used -> tho7e.exe called for spellcard.eng which was missing -> Unhandled Exception Error occurred.
I'm concerned about my English since it's not my native language.