In case of the german URL-Keys (projekte/projekt-a), the /de is missing from the generated link. In case of the english URL-Keys (projects/project-a) which is also the uid of the page, the correct german link is generated.
A solution is of course to use only the default language URL-Keys when linking pages, but for clients, this might not be very intuitive. Do you guys know of a solution to this problem?