l10n: de.po: translate 54 new messages

Translate 54 new messages came from git.pot update in
c138af56 (l10n: git.pot: v1.8.3 round 1 (54 new, 15 removed)).

While at there, fix some small issues.
Signed-off-by: NRalf Thielow <ralf.thielow@gmail.com>
Acked-by: NThomas Rast <trast@inf.ethz.ch>