The consensus seems to be—no you can't. The reason I assumed you could ("Yes I can") was that Calibre uses dict.org for its reader*, and dict.org has all kinds of other dictionaries you can choose on their site (
http://www.dict.org/), e.g. the German-English Freedict Dictionary. It would surely be a simple matter to make the switch if one knew how (maybe through style sheets??).
*From the Calibre FAQ "You can lookup the meaning of words in the current book by right clicking on a word. calibre uses the publicly available dictionary server at dict.org to lookup words. The definition is displayed in a small box at the bottom of the screen."