/your/install/path/words.R.wom. By default, this is /var/lib/philologic/words.R.wom
It's simply a two-fielded, tab-delimited, newline-separated data file, with the surface form on the left and the search form on the right. You enter the term on the left, and unbeknownst to the end user, the search is actually run on the second form on the right. crapser greps through it to find the matches.
The default version that shows up there has the same thing in both columns and doesn't do any translating.... you need to make your own (see make_wom_words.R.pl).