Hi,
1. No. It needs to be done manually.
2. Go in the Languages tab of the configuration and click on the edit icon of your language, and then on the "load latest from server" button and then save.
3. You should get a notification that your language file is not up to date if you go in your HikaShop configuration page, unless you clicked before on the "I know, don't remind me" link of the message.
4. No it doesn't. It's always there, even if you already have the latest version. That way, if you change the language file, you can always go back to the official latest version.
5. No. For HikaSerial, all the language files are included by default in the installation package so the are updated automatically, which is not possible for HikaShop as it has too many language files and that they are quite big and it would make the installation package too big.