It won't be replacing Dictionary.app, but it does look like a a good subject for learning Go.
Unfortunately, though, its a bit complex to even try to use the local dictionary: https://github.com/Rican7/define/issues/1
I appreciate you taking the time to look at the source. I used this, like I do most of my side projects, as an opportunity to learn a few things.
I wanted a generic abstraction so that multiple sources could be used. Originally I didn't want or think of the need to support multiple sources, but it quickly became apparent how necessary that was when looking at the quality of free vs paid vs personal-key-required sources.
In any case, thanks for checking it out!