Setting different languages for the back-end and storefront
Error: If you not able to switch the frontend to German although the settings in admin are correct.
Or you need to set the part that is visible to the customers in one language (German) and the backend (Dashboard) in another language (English).
Solution:
In this case, may be you have missed some steps.
This is the document that we consulted online, hope it can help you to understand the problem.