I'm internationalizing a Pinax website and I've encountered this piece of code in a template:

<input type="submit" value="{% trans "invite" %}"/>
{% blocktrans %}{{ other_user }} to be a friend.{% endblocktrans %}

The message ids for this code would be two separate blocks: "invite" and " %{other_user}s to be a friend". Both offer very little in terms of context and make the translators job difficult. Correct, in my point of view, would be the more convoluted form of:

{% blocktrans %}
<input type="submit" value="invite"/>
{{ other_user }} to be a friend.
{%  endblocktrans %}

This implies that the translators know enough HTML to notice that the value attribute needs to be translated, but the end result is a lot more flexible and provides real context to them.

TL;DR: don't split paragraphs into separate translation units. It's a NO-NO.

UPDATE: I have found what is probably the worst example of how to create a translatable template. Remember, don't assume the English language resembles anything like another language.

{% trans "edited by user" %} {{ obj.editor.username }} {% trans "at"%} {{ obj.modified|date:"H:i" }}

This should be done this way:

{% blocktrans with obj.editor.username as editor_username and obj.modified|date:"H:i" as obj_modified
edited by user {{ editor_username }} at {{ obj_modified }}
{% endblocktrans %}