In Mountain Lion the OS X internal dictionary that you can fire up by right clicking a word, or by using the dashboard widget, seems to have gained at least one new language: German. Since German is my main language, but I also read a lot of English texts and website, I'd like to switch the dictionary languages on the fly. How can I do this?
Whichever dictionaries you enable in the Dictionary app's preferences will be available in the dictionary popover. You could either change these whenever you want to switch, or perhaps just leave both languages enabled all the time.
jtbandes' method is great if Apple includes the language that you desire to switch to, but note that you can also download external dictionaries with the ".dict" extension. Here are a couple to get you started:
Here's what you do once you've downloaded the dictionaries you want:
You can check out a bunch more dictionaries at this link: http://speedofmac.com/adding_dictionaries.html.
Hope this helps!