l10n: de.po: prepare de.po for new round of translation

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
l10n: de.po: translate 39 new messages

Translate 39 new messages came from git.pot update in
c138af5 (l10n: git.pot: v1.8.3 round 1 (54 new, 15 removed)).

While at there, fix some small issues.

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Acked-by: Thomas Rast <trast@inf.ethz.ch>