Yes it is for private Use legal, have a look here:
http://www1.dict.cc/translation_file_request.php
(for example a perfect German - English - German Dictionary what i use on my jailbroken iPad2 as global dictionary with 500000 words)
http://lipflip.org/articles/dictcc-dictionary-plugin
That means, the Developer of Marvin can give us a interface without the Dictionarys. The Dictionary's by them self you have to install separately. You can find hundreds of them from other guys who have it already done the work for you. Otherwise you have the possibility to use this Tool:
http://code.google.com/p/mac-dictionary-kit/
To convert Stardict Dictionarys. Most of this Dictionarys have the GPL as license or are free. I think this will be a good start :-)
http://abloz.com/huzheng/stardict-dic/freedict.de/
(go one level UP to see tons of more dictionarys)
Ciao Marco