Filtration of posts by language in jekyll blog on GitHub Pages

[irl]

I add to my blog the translation of posts into different languages. I've been doing the translation, but I can't figure out how I can display messages in one language on the post page. To date, I have duplicated articles in different languages. For starters, this is my code that connects attributes.

{% if page.layout != 'index' %}
    {% assign posts=site.posts | where:"lang-ref", page.lang-ref | sort: 'lang' %}
    {% if posts.size == 0 %}
        {% assign posts=site.pages | where:"lang-ref", page.lang-ref | sort: 'lang' %}
    {% endif %}
    {% if posts.size > 1 %}
    <footer class="lang-options">
        <em>Read this page in a different language:</em>
        <ul>
        {% for post in posts %}
        <a href="{{ site.base-url }}{{ post.url }}" class="{{ post.lang }}" title="View in {{post.lang}}">{{ post.lang }}</a>
        {% endfor %}
        </ul>

That's how I put the posts on the page:

{% for post in paginator.posts %}
<article class="post-preview">
  <a href="{{ post.url | prepend: site.baseurl | replace: '//', '/' }}">
    <h2 class="post-title">{{ post.title }}</h2>
    {% if post.subtitle %}
    <h3 class="post-subtitle">{{ post.subtitle }}</h3>
    {% else %}
    <h3 class="post-subtitle">{{ post.excerpt | strip_html | truncatewords: 15 }}</h3>
    {% endif %}
  </a>
  <p class="post-meta">Posted by
    {% if post.author %}
    {{ post.author }}
    {% else %}
    {{ site.author }}
    {% endif %}
    on {{ post.date | date: '%B %d, %Y' }} &middot; {% include read_time.html content=post.content %}
  </p>
</article>

That's how I try to filter messages on the page. The code is intended to eject messages in the language of the current page.

 <article class="post-preview">
  {% assign posts = site.posts | where:"lang", page.lang %}
  {% for post in posts %}
  <a href="{{ post.url | prepend: site.baseurl | replace: '//', '/' }}">
    <h2 class="post-title">{{ post.title }}</h2>
    {% if post.subtitle %}
    <h3 class="post-subtitle">{{ post.subtitle }}</h3>
    {% else %}
    <h3 class="post-subtitle">{{ post.excerpt | strip_html | truncatewords: 15 }}</h3>
    {% endif %}
  </a>
  {% endfor %}

But filtering doesn't work, and I get a mistake.

Liquid Exception: undefined method `split' for nil:NilClass in posts/index.html

[chanisef]

Maybe someone also faced this filtration problem. That's how I decided to:

{% assign sorted-posts = site.posts | where: "lang", page.lang %}
{% if sorted-posts.size > 0 %}
{% for post in sorted-posts %}

Same grading on the front page:

{% assign sorted-posts = site.posts | where: "lang", "en" %}
        {% for post in sorted-posts limit: 5 %}

It's just that all the pages should be able to use the language and remove the posts in the front page only.