Separate names with a comma.
Done it - I downloaded the package files from https://packages.ubuntu.com/bionic/language-pack-de, and went through the Settings procedure...
Thanks for trying, but I got the same problem. The language packages weren't found. :-(
I'm trying to install German as a second language in Ubuntu 18.04, but every time I do, I get a 'Failed to fetch HTTP' error. I'm definitely on...