Return a list of all used languages for this page.

def get_languages(self):
        """
        Return a list of all used languages for this page.
        """
        if self._languages:
            return self._languages
        self._languages = cache.get(self.PAGE_LANGUAGES_KEY % (self.id))
        if self._languages is not None:
            return self._languages

        languages = [c['language'] for
                            c in Content.objects.filter(page=self,
                            type="slug").values('language')]
        # remove duplicates
        languages = list(set(languages))
        languages.sort()
        cache.set(self.PAGE_LANGUAGES_KEY % (self.id), languages)
        self._languages = languages
        return languages