Fix regression in Python 3.

product/Localizer/MessageCatalog.py

@@ -660,6 +660,7 @@ class MessageCatalog(LanguageManager, ObjectManager, SimpleItem):
         messages = self._messages
 
         # Load the data
+        if isinstance(data, bytes):
             data = data.decode("utf-8")
         po = polib.pofile(data)
         encoding = to_str(po.encoding)