Quickly translate a word from English

You could look for additional dictionaries for use in Dictionary.app (included with OS X). Just like my Wikipedia dictionary (don't know if it came with the vanilla OS X installation) loads data from the web, so could your translation service. It seems to be not possible to enhance the Cmd-Ctrl-D "quick dictionary" with such a custom dictionary, though.

You can enable debugging/developer mode for Dashboard which allows you to drag widgets out of the "Dashboard view" onto the normal desktop, so you always have the widget visible.

You can look into creating a custom Text service for the Services menu (select a word in the browser and look under Application -> Services -> Text. In System Preferences -> Keyboard -> Keyboard Shortcuts -> Services you can even set keyboard shortcuts for such commands.

The custom service can be easily created using Automator.app, I just tried:

• Create a new Automator document based on the "Service" workflow
• Add a single action: Run Shell Script with the input passed as arguments, and the command open http://dict.cc/?s=$1 (or whatever website you use for translating)
• Save as Translate to German (in my case it's German)
• (Optionally) Assign a keyboard shortcut in System Preferences
• Use it

There is a translate dashboard widget that comes with Mac OS X, since version 10.4 I believe. It includes spanish translations.