copied everything from multilingual example

accueil.html

+---
+layout: default
+ref: index
+lang: fr
+---
+
+<div class="home">
+
+  <h1 class="page-heading">Articles</h1>
+
+  <ul class="post-list">
+    {% assign posts=site.posts | where:"lang", page.lang %}
+    {% for post in posts %}
+      <li>
+        <span class="post-meta">{{ post.date | date: "%b %-d, %Y" }}</span>
+
+        <h2>
+          <a class="post-link" href="{{ post.url | prepend: site.baseurl }}">{{ post.title }}</a>
+        </h2>
+      </li>
+    {% endfor %}
+  </ul>
+
+  <p class="rss-subscribe">suivre le <a href="{{ "/flux.xml" | prepend: site.baseurl }}">flux RSS</a></p>
+
+</div>

css/main.scss

+---
+# Only the main Sass file needs front matter (the dashes are enough)
+---
+@charset "utf-8";
+
+
+
+// Our variables
+$base-font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
+$base-font-size:   16px;
+$base-font-weight: 400;
+$small-font-size:  $base-font-size * 0.875;
+$base-line-height: 1.5;
+
+$spacing-unit:     30px;
+
+$text-color:       #111;
+$background-color: #fdfdfd;
+$brand-color:      #2a7ae2;
+
+$grey-color:       #828282;
+$grey-color-light: lighten($grey-color, 40%);
+$grey-color-dark:  darken($grey-color, 25%);
+
+// Width of the content area
+$content-width:    800px;
+
+$on-palm:          600px;
+$on-laptop:        800px;
+
+
+
+// Use media queries like this:
+// @include media-query($on-palm) {
+//     .wrapper {
+//         padding-right: $spacing-unit / 2;
+//         padding-left: $spacing-unit / 2;
+//     }
+// }
+@mixin media-query($device) {
+    @media screen and (max-width: $device) {
+        @content;
+    }
+}
+
+
+
+// Import partials from `sass_dir` (defaults to `_sass`)
+@import
+        "base",
+        "layout",
+        "syntax-highlighting"
+;

feed.xml

+---
+layout: null
+lang: en
+---
+<?xml version="1.0" encoding="UTF-8"?>
+<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
+  <channel>
+    <title>{{ site.title[page.lang] | xml_escape }}</title>
+    <description>{{ site.description[page.lang] | xml_escape }}</description>
+    <link>{{ site.url }}{{ site.baseurl }}/</link>
+    <atom:link href="{{ "/feed.xml" | prepend: site.baseurl | prepend: site.url }}" rel="self" type="application/rss+xml"/>
+    <pubDate>{{ site.time | date_to_rfc822 }}</pubDate>
+    <lastBuildDate>{{ site.time | date_to_rfc822 }}</lastBuildDate>
+    <generator>Jekyll v{{ jekyll.version }}</generator>
+    {% assign posts=site.posts | where:"lang", page.lang %}
+    {% for post in posts limit:10 %}
+      <item>
+        <title>{{ post.title | xml_escape }}</title>
+        <description>{{ post.content | xml_escape }}</description>
+        <pubDate>{{ post.date | date_to_rfc822 }}</pubDate>
+        <link>{{ post.url | prepend: site.baseurl | prepend: site.url }}</link>
+        <guid isPermaLink="true">{{ post.url | prepend: site.baseurl | prepend: site.url }}</guid>
+        {% for tag in post.tags %}
+        <category>{{ tag | xml_escape }}</category>
+        {% endfor %}
+        {% for cat in post.categories %}
+        <category>{{ cat | xml_escape }}</category>
+        {% endfor %}
+      </item>
+    {% endfor %}
+  </channel>
+</rss>

flux.xml

+---
+layout: null
+lang: fr
+---
+<?xml version="1.0" encoding="UTF-8"?>
+<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
+  <channel>
+    <title>{{ site.title[page.lang] | xml_escape }}</title>
+    <description>{{ site.description[page.lang] | xml_escape }}</description>
+    <link>{{ site.url }}{{ site.baseurl }}/</link>
+    <atom:link href="{{ "/flux.xml" | prepend: site.baseurl | prepend: site.url }}" rel="self" type="application/rss+xml"/>
+    <pubDate>{{ site.time | date_to_rfc822 }}</pubDate>
+    <lastBuildDate>{{ site.time | date_to_rfc822 }}</lastBuildDate>
+    <generator>Jekyll v{{ jekyll.version }}</generator>
+    {% assign posts=site.posts | where:"lang", page.lang %}
+    {% for post in posts limit:10 %}
+      <item>
+        <title>{{ post.title | xml_escape }}</title>
+        <description>{{ post.content | xml_escape }}</description>
+        <pubDate>{{ post.date | date_to_rfc822 }}</pubDate>
+        <link>{{ post.url | prepend: site.baseurl | prepend: site.url }}</link>
+        <guid isPermaLink="true">{{ post.url | prepend: site.baseurl | prepend: site.url }}</guid>
+        {% for tag in post.tags %}
+        <category>{{ tag | xml_escape }}</category>
+        {% endfor %}
+        {% for cat in post.categories %}
+        <category>{{ cat | xml_escape }}</category>
+        {% endfor %}
+      </item>
+    {% endfor %}
+  </channel>
+</rss>

index.html

+---
+layout: default
+ref: index
+lang: en
+---
+
+<div class="home">
+
+  <h1 class="page-heading">Posts</h1>
+
+  <ul class="post-list">
+    {% assign posts=site.posts | where:"lang", page.lang %}
+    {% for post in posts %}
+      <li>
+        <span class="post-meta">{{ post.date | date: "%b %-d, %Y" }}</span>
+
+        <h2>
+          <a class="post-link" href="{{ post.url | prepend: site.baseurl }}">{{ post.title }}</a>
+        </h2>
+      </li>
+    {% endfor %}
+  </ul>
+
+  <p class="rss-subscribe">subscribe <a href="{{ "/feed.xml" | prepend: site.baseurl }}">via RSS</a></p>
+
+</div>

jekyll-multilingual.gemspec

+Gem::Specification.new do |s|
+  s.name     = 'jekyll-multilingual'
+  s.version  = '3.1.6'
+  s.license  = 'MIT'
+  s.summary  = 'Multilingual version of the Jekyll default theme'
+  s.author   = 'Sylvain Durand'
+  s.email    = 'github@durand.tf'
+  s.homepage = 'https://www.sylvaindurand.org/making-jekyll-multilingual/'
+  s.files    = `git ls-files -z`.split("\x0").grep(%r{^_(includes|layouts)/})
+end

robots.txt

+User-agent: *
+Allow: /

sitemaps.xml

+---
+layout:
+permalink: /sitemaps.xml
+---
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
+  {% for post in site.posts %}
+    {% if post.id contains "404" %}{% else %}
+      <url>
+        <loc>{{site.base}}{{ post.url }}</loc>
+        {% assign versions=site.posts | where:"ref", post.ref %}
+        {% for version in versions %}
+          <xhtml:link rel="alternate" hreflang="{{ version.lang }}" href="{{site.base}}{{ version.url }}" />
+        {% endfor %}
+        <lastmod>{{ post.date | date_to_xmlschema }}</lastmod>
+        <changefreq>weekly</changefreq>
+      </url>
+    {% endif %}
+  {% endfor %}
+  {% for page in site.pages %}
+    {% if page.id contains "404" %}{% else %}
+      <url>
+        <loc>{{site.base}}{{ page.url }}</loc>
+        {% assign versions=site.pages | where:"ref", page.ref %}
+        {% for version in versions %}
+          <xhtml:link rel="alternate" hreflang="{{ version.lang }}" href="{{site.base}}{{ version.url }}" />
+        {% endfor %}
+        <changefreq>weekly</changefreq>
+      </url>
+    {% endif %}
+  {% endfor %}
+  </urlset>