Tim,
some of them are preparations for the change to XHTML in version 1.5. parentlanguage defines which language should be used if in your chosen language a string isn't translated. Normaly this is english. In German we have two versions de and de_du. The de_du version is not complete. Now we can link from the de_du version to the complete de-Version if a string is missing in the de_du version. The user see a german text and not a english text.