l10n: de.po: correct translation of "completed" after resolving deltas

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
[DRAFT] l10n: de.po: translate 37 messages

Translate 37 new messages came from git.pot update in 3631678
(l10n: git.pot: proposed updates for v2.0.0 (+37)).

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>