How do you switch dictionaries used for the "Tap with three fingers" lookup? Using Mountain Lion, I want to add or change the dictionary used when I tap with three fingers to define a word. The goal is to use a spanish-english dictionary in place of the english dictionary.
You select the dictionary in Dictionary.app > Preferences.

For info about adding dictionaries, see

http://m10lmac.blogspot.com/2011/12/more-dictionaries-for-dictionaryapp.html