Installing extended offline dictation fails without error
Please follow the instructions to remove a ?.plist file (it will be recreated automagically once you restart the system).


Press these keys simultaneously: shift+⌘+G
In the dialog box that appears after step 1, copy and paste the following 



  ~/Library/Preferences



Hit return/enter
You will be presented with a ton of files, but you should be able to find the 



  com.apple.assistant.plist



Move it to the Trash and Empty your Trash
Open Dictation &amp; Speech preference panel and turn off dictation
Restart your computer
Open Dictation &amp; Speech preference panel, turn on dictation
Select Enhanced Dictation
Download should begin and install correctly


I hope this helps.

The file you are downloading is 750 meg, so it might take a while.