I've used anki for learning German a few years ago and it really worked for me - I was able to learn a few (<500) hundred new words in about 2 months. Not bad at all for a person with no language talent. Although, I've stopped learning the language 3 years ago, I still understand German texts, thanks to anki.
When using anki, a browser extension, enabling creation of flashcards, from the content found on the Internet, is handy. I find a website fluentcards.com and its browser extension, quite OK. Unluckily, it seems not to be maintained and there is no official add-on for Firefox, but you can build it from the github repo [1] and it will work on FF, too.
The webpage also allows export of the words saved on Kindle to the anki.
There are also a few extensions for learning Japanese specifically. [2] seems to be the most popular.