For some reasons BeautifulSouf does not convert entities with convertEntities=["xml","html"], use convertEntities=BeautifulSoup.ALL_ENTITIES instead.
{% extends "site/layout/base.html" %}
{% load i18n %}
{% block content %}
<h1>500: {% blocktrans %}Server Error{% endblocktrans %}</h1>
{% blocktrans %}An error has occurred.{% endblocktrans %}
{% endblock %}