Hi, Dictionaries for the Dictionary application, widget, contextual menu and Services menu are stored in /Library/Dictionaries. The (unrelated) dictionaries for spell checking are in /System/Library/Services/AppleSpell. It is probably technically possible for someone to create more Dictionary.app dictionaries, but no one seems to, and Apple doesn't have any for download. So, you would need to switch to an alternative dictionary app (sigh).
James.
