'All official messages (i.e. from main database) for the given lang'

def official_messages(self, lang):

    lang_id = self.language_ids[lang]
    try:
        return self._sources[lang_id]
    except AttributeError:
        self._sources = {}
        for message in self.yield_source_messages():
            self._sources.setdefault(message.language_id, []).append(message)
        self._sources = dict(((k, tuple(merge_adjacent(v))) for (k, v) in self._sources.items()))
        return self.official_messages(lang)
    except KeyError:
        assert (lang not in self.official_langs)
        return ()
