jekyll-theme-chirpy/_includes/lang.html

{% comment %}
  Detect appearance language and return it through variable "lang"
{% endcomment %}
{% if site.data.locales[page.lang] %}
  {% assign lang = page.lang %}
{% elsif site.data.locales[site.lang] %}
  {% assign lang = site.lang %}
{% else %}
  {% assign lang = 'en' %}
{% endif %}