IMPORTANT! It seems that Duolingo's website has been updated to break this script. However, as Duolingo releases new version of their site to a few users at a time, it is possible the script still works for you. Therefore I've edited the script to work if it can, and tell you to remove it if it can't.
This will add a "Ninja Button" to the language list on the toolbar. The Ninja Button will allow you to access Duolingo's Japanese course from the web.
Be aware the the web version of the Japanese course is not finished and has more errors than the iOS version.
If you try to access some parts of the website, Duolingo will present you with a page telling you that Japanese for the web is not yet ready, and it will prompt you to select a new language to learn. While the Ninja Button *will* appear on that page, it won't actually work. You will need to select a new language to learn and then from the homepage of that language, press the Ninja Button again.