How to add wiktionary as a source to the default dictionary.app?
You can use DictUnifier to convert any StarDict format dictionary into a mac format dictionary.  You can then download wiktionary in StarDict format here:

http://www.dictinfo.com/

Extract the 7z file with Keka or similar.  You will get three files, a .ifo, a .dictz, and a .idx.

Drag the resulting .ifo file onto the DictUnifier drop location and it will convert and import it into your Dictionary.  You can then go into preferences and check the new dictionary and drag it into the right position for how you want results.

Now you have offline wiktionary access in every language that is available at that dictinfo site (Gobs of them!!).