Comment 18 for bug 568826

Alternative solution that shows how much file space your are using, but only in English language:
        self.usage_label.set_text(
            _("%(USED)0.1f %(TYPE)s Used (%(PERCENT)0.1f%%)") % {
                'USED' : real_used,
                'TYPE' : real_type,
                'PERCENT' : percent })
        self.usage_graph.set_fraction(percent / 100)

It is caused by translating names/identifiers found inside %(...). I am currently trying to find out how to submit a correct translation for nb (Norwegian, bokmål).