I am not exactly sure why Apple included a Japanese dictionary in OSX 10.5. If you would like to see it, here is how you enable it. By using these directions you can also add or remove the wikipedia and apple sections from the dictionary as well.
Apple included a bunch of new dictionaries plug-ins that can be integrated into the main Leopard dictionary.
You can add, remove, or rearrange these dictionaries through the following menu…
Add check marks by the ones you want enabled.
Remove check marks by the ones you don’t want displayed.
You can drag and drop each one to place it in the order you want it displayed.
