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>404: {% blocktrans %}Page not found{% endblocktrans %}</h1>
{% blocktrans %}The requested page could not be found.{% endblocktrans %}
{% endblock %}