[x/tour] content/welcome.article: add German translation
[x/tour] content/welcome.article: add German translation

Reported on golang-nuts:
https://groups.google.com/d/topic/golang-nuts/pwbBQPkk2CQ/discussion

While we're here, make the use of dashes consistent.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/197460043
X-Tour-Commit: 446eef14860582bcb1874118339f1a2c41d477e3

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website