Hi all, I am the person who created the Appdate file for Widelands.
The reason for the warning: tag-missing : <translation> not specified is that after extensive RTFM at the time I had no idea how to specify this, so I decided to ship my own and add the translations directly to the xml nodes as per the spec. They are provided by the translators as JSON and I then run a script to generate them both for appdata and the .desktop file. If there is a smarter way of doing, this, I'm all ears. Cheers GunChleoc