1. languages files are not included with download, you will need to download the language that you need, this is to avoid using user server space.
2 I think you are downloading the wrong file, check a few languages and all include the .po file.
3. Not all languages are up to date, contributors have not provided update for the lates version, I think the Spanish version is complete.
4. This could be because of what I mentioned on item 3.
5. Thank you for your interest on providing the Dutch translation, you can send it to [email protected]
Thank you for your feedback.