Update German messages.json to add missing i18n

[ottlinger]

First time contributor checklist:

• I have read the README and Contributor Guidelines
• I have signed the Contributor Licence Agreement

Contributor checklist:

• My contribution is not related to translations.
• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest main branch
• A npm run ready run passes successfully (more about tests here)
• My changes are ready to be shipped to users

Description

Added i18n keys for not translated contents and harmonized existing i18n to use the same words, fix minor formatting issues (whitespace)

[scottnonnenberg-signal]

Thanks for taking the time to look in detail at our German strings! However, we only update the en/messages.json file in this repository, and for every release we download the latest localized strings from our translation system. I've passed your German string updates to our localization folks, and I am going to close this PR.

[maba4891]

Hey,
German Language Manager here! I'll review your suggested changes and integrate them where necessary. Thanks for your contribution @ottlinger!

[ottlinger]

Alright, I'll leave my fork open and look for changes in the upstream project. Thanks.