GDict

With GDict, make Google Dictionary more convenient to use on your desktop, with compact&clean interface. You can lookup words in browser, pdf, doc even video easily with a nicely looking mini floating window which won't interrupt your work, and don't need to open browser or other heavy dictionary app to lookup words any more.

GDict support Arabic, Chinese, French, German, Portuguese, Russian and Spanish at now.

It's very easy to use, you can easily bring it to frontend with Alt-D, you can also change the hotkey to anyone you like in Preferences. If you like, you can also enable "Lookup with Google Dictionary" in your services panel, and then you can lookup the current selected words in any app with right-click menu or a hotkey you set (the default is ⌘-S).