Greasy Fork is available in English.

Access Japanese on Duolingo

To enable Japanese course on Duolingo web obviously. You need to be on the Discussion page to see the Japanese menu entry in your learning menu.

// ==UserScript== 
// @name Access Japanese on Duolingo  
// @version 0.1 
// @description To enable Japanese course on Duolingo web obviously. You need to be on the Discussion page to see the Japanese menu entry in your learning menu.
// @author Goshinki 
// @match https://www.duolingo.com/discussion* 
// @grant none 
// @require http://code.jquery.com/jquery-latest.js 
// @namespace https://greasyfork.org/users/143845
// ==/UserScript==

(function() { 
    'use strict';
    $(
        $(document.createElement('li')).attr({
            'class': 'language-choice',
            'data-value': 'ja'
        })).append(
            $(document.createElement('a')).attr('href', 'javascript:;').append(
                $(document.createElement('span')).attr('class', 'flag flag-svg-micro flag-ja'),
                $($(document.createElement('span')).attr('data-value', 'ja')).text('Japanese')
            )
    ).insertBefore(
        $(".language-choice").parent().children('.divider')
    );
})();