Thanks Ivan! I also had to export PYTHONPATH with my project's root dir to make it work.
But in generated django.po there is no translation strings for file: contrib/comments/templatetags/comments.py I noticed that in this file you have long strings and which include lines like: <p><input type="submit" name="preview" value="{% trans "Preview comment" %}" /></p> (line 45 for example). Generation of .po files doesn't extract "Preview comment" strings (it uses xgettext as I noticed). You have to include those kind of strings by hand in your po file. Is this a bug?