From the developer : You have been looking for Mac OS X dictionary, which could translate any word from any language, for a long time?
Congratulations! You have just found it!
We are glad to present you TranslateIt! for Mac!
TranslateIt! for Mac - is the multilingual dictionary that allows looking up words on the fly!
Just hover the pointer over an unfamiliar word (in any Cocoa Application), and you will see its translation in a pop-up window. Instantly.
A Free version is also available