LANGUAGE_CODE is a string, so you just need to enquote your comparison value like this:
{% if request.LANGUAGE_CODE == 'en' %}
<h1>English</h1>
{% endif %}
check also the ifequal tag
{% ifequal request.LANGUAGE_CODE 'en' %}
...
{% endifequal %}
a bit more: the if and ifequal on strings are case sensitive, so you may want to be sure you're matching the correct case (maybe applying the |lower filter to both arguments)