several small improvements to messages.de.yml

What does this MR do?

corrects/improves several text strings in messages.de.yml (mainly according to weblate comments)

How confident are you it won't break things if deployed?

quite a lot, this MR includes only textual changes and no deeper programming

Links to related issues

https://hosted.weblate.org/translate/foodsharing/foodsharing-messages/de/?q=has:comment#comments

How to test

see diff

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• added to the next milestone (see https://gitlab.com/foodsharing-dev/foodsharing/-/milestones, unless it has a "for:Dev" label)
• added an entry to CHANGELOG.md