OS X has a three-finger feature that translates or explains (depending on the dictionary) any word in any text. It has english dictionary by default plus wikipedia.
- 20mb http://www.macupdate.com/app/mac/10376/english-german-dictionary. This is far too insufficient.
- dict.cc integrated 600mb http://lipflip.org/articles/dictcc-dictionary-plugin. This has a lot more words. Also "lookup online" feature that goes to dict.cc.
Dictionaries are installed either in /Library/Dictionaries or in /Users/<UserName>/Library/Dictionaries.