I can confirm that after putting a French-English dictionary in my Kindle Touch documents directory with the USB cable, the Kindle used this dictionary when looking up French words.
I got the dictionary from
http://blog.mikeasoft.com/2011/01/05...or-the-kindle/
The book is Le Comte de Monte Cristo in mobi format from
http://www.gutenberg.org/
Hope this helps as much as this thread helped me!