Apply bitnapper's German translation fixes

[cdrini]

Closes #3729 . This adds just the German additions to the messages.po file ; any changes to the strings were too difficult to untangle for me, so I just chose to use the default where available. This added ~1% coverage to our German translations! Some of the added strings are no longer part of our HTML unfortunately, but I they'll be handled correctly next time we regen the .po files.

Technical

I found some useful commands! Here's what I ran. I downloaded the final messages.po from #3729 , and then used a gettext function called msgcat to concatenate the two po files, choosing translations from the current master branch in cases where there was a conflict:

docker-compose run -u root --rm --no-deps web bash -c "
    apt-get update && apt-get install -y gettext
    su openlibrary -c '
        msgcat --width=83 --use-first openlibrary/i18n/de/messages.po openlibrary/i18n/de/messages2.po > openlibrary/i18n/de/merged.po
    '
"

I then moved the merged file over to messages.po, and did some manual cleanup where things were funny or it had introduced arbitrary newlines.

Testing

Screenshot

Stakeholders

@bitnapper @jimchamp

[jimchamp]

Lgtm!