Here is an easy way to handle i18n in a Jekyll website.

en.yml

# _data/en.yml

header:
  title: "Hello world!"

fr.html

<!-- index.html -->

---
lang: fr
---

{% include header.html %}

fr.yml

# _data/fr.yml

header:
  title: "Bonjour le monde!"

header.html

<!-- _includes/header.html -->

<h1>{{ site.data[page.lang].header.title }}</h1>

index.html

<!-- index.html -->

---
lang: en
---

{% include header.html %}