Add german language (de-DE)

[dominique-mueller]

Relates to #183

Description

This PR introduces german (de-DE / de) as an additional language. Yay!

In addition, it fixes various i18n-related issues, such as

• locale data other than english not being provided to dayjs
• dates not being updated properly after changing language
• tab switching no longer working with new languages / after switching language
• prop types errors in console (pre-existing)

Checklist

• I have run yarn format
• I have run yarn eslint and fixed any issues
• This PR does not add any errors to the console
• I have added this change to the changelog

A build will automatically be run by GitHub actions when any changes are made on this PR. This must complete successfully before merging.

Screenshots or videos

Click to expand

[dominique-mueller]

Not sure how to extend the changelog before an actual release, feel free to update the PR as required :)

[davwheat]

Thanks so much for your work on this! It's fantastic!

I've rebased your branch onto the latest commits (after merging the other language additions) and have popped it into the changelog for you :)