class Message(models.Model):
    subject = models.CharField(_("Subject"), max_length=120)
    body = models.TextField(_("Body"))

    st = loader.get_template("mails/internal_on_reg_sj.html")

    msg = Message(
                ...
                subject = st.render(Context({"user":user})),
                ...)

    msg = Message(
                ...
                subject = st.render(Context({"user":user})).encode('utf-8'),
                ...)

'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128)

{% load i18n %}{% trans "Hello" %} {{user.username}}!

   body = models.TextField(_("Body")) примерно тоже самое хавается.