Kris, the dict format itself is not encumbered in any way.
Individual dictionaries, of course, can be but there are a lot of GPL'ed dictionaries. If an App under a proprietary licence can use them, I haven't researched.
You'll find an extensive collection of free dict format dictionaries on http://freedict.org/en/
Since Debian and other major Linux distros include those, they're certainly safe to use in general. Especially Debian and Fedora are very thorough in their legal investigations and they ship them.
*If* you really want to include additional dictionary support, dict format support would most likely make sense.