I can't run it right now as I'm not on a GNUstep-capable device. But maybe this will help:
Looks like besides reading local files, the app talks to a service at
dict.org:
Dictionary preferences and maybe even dictionaries seem to live in ~/GNUstep/Library/DictionaryReader **
Finally, why not imitate the layout of the Resources directory and drop in the French dictionary next to jargon.dict?
(Again, these are blind suggestions, I don't know what the app looks like.)
** Nitpick: this is a bug, but not related to what you are seeing (grayed-out prefs item). There is no reason why the home library folder should be in ~/GNUstep/Library, it's just the current default.