VIM spell files seem to be in some binary format. Is it possible (and easy) to convert one back to a plain text word list?
Yes, you can use
:spelld[ump] Open a new window and fill it with all currently valid
words. Compound words are not included.
Note: For some languages the result may be enormous,
causing Vim to run out of memory.
Vim will use the language file that you set with the