In OS X 10.5, the Dictionary has been significantly improved over the last version. Now its internals allows other content sources, such as Japanese dictionaries, Apple terms, and Wikipedia access.

Dictionary chooses the best-suited language for Wikipedia content based on the language of the actual system, but it is possible to support multiple language sources at same time. To make this happen, simply:

1. Open Dictionary.app in Applications Folder.
2. Select Preferences from the Dictionary Menu.
3. Select Wikipedia and a list of languages will appear.
4. You can select how many languages you want.

One more thing ... by studying the new Dictionary structure, it looks like it may be possible for third party developers to integrate new dictionaries into the system in future. The dicionaries are no longer hard-coded into the application itself.