Add another tag not to be translated
{% load i18n %}
-{% comment %} Translator: This email is sent to users when they receive a comment on their application. Don't translate {{ partner }} or {{ app_url }}.{% endcomment %}
+{% comment %} Translator: This email is sent to users when they receive a comment on their application. Don't translate {{ user }}, {{ partner }} or {{ app_url }}.{% endcomment %}
{% blocktrans trimmed with partner=app.partner user=app.user %}
Dear {{ user }},
