Might be clearer to do:
```python
tweet_action = u"http://twitter.com/intent/tweet?text={text}".format(
    text=_("I just registered for {number} {title} through {account}: {url}").format(
        number=course.number,
        # etc
    )
).replace(u" ", u"+")
```

By defining the tweet text inline, it makes it easier to see where the replacement tokens are in the format string. Also, by moving the space-to-plus conversion to the end, you guarantee that you the string formatting isn't inserting any spaces into the converted URL.