I know this is a slightly old pots but ....
There isn't a built-in Japanese/English dictionary to use with the Kindle Paperwhite, specifically I couldn't find one that could be used as the default for automatic word lookups.
That being said I went and created one myself and published it on Amazon.
If you are interested the ASBN for it is B00AKIUDAY
You can find it here:
www.amazon.co.jp/dp/B00AKIUDAY/
www.amazon.com/dp/B00AKIUDAY/
If it is not available in your country feel free to send me a private email/message and I will see if I can get it published in your country.