When I wrote “Replace the existing German locale files with the new ones in your Discourse instance.” I should have mentioned that this isn’t as easy as it sounds. 
You will need a development environment in order to follow Add a new locale from plugin and you will need https://github.com/discourse/translations-manager to pull translations from Transifex and apply some transformations to the locale files.
Unfortunately this is a non-trivial task if you aren’t familiar with the tools. I’d really like to improve the whole process of creating language variants, but that’s probably a project for next year. However, I can give you a hand if you are willing to wait until the end of June.