Apple - How to switch language of OS X dictionary?

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:

• dict.cc Dictionary Plugin (German-English)
• English to Latin Dictionary and Latin to English Dictionary
• French Dictionary

Here's what you do once you've downloaded the dictionaries you want:

1. Navigate to /Library/Dictionaries by doing this: Finder -> Go -> Go To Folder -> /Library/Dictionaries

2. Move the dictionaries from your Downloads folder to /Library/Dictionaries

3. Open up Dictionary and go to Dictionary -> Preferences and select the dictionaries you just added.

You can check out a bunch more dictionaries at this link: http://speedofmac.com/adding_dictionaries.html.

Hope this helps!

It is also possible to change dictionary priority by dragging your dictionary to the top.

And if you need a dictionary not provided by Apple then you can convert other formats to AppleDict as described here.