Yes, your web.config forces the thread to always execute as German. This is not advisable, you should keep that globalization section as it was which was configured for auto detection of the browser language with fallback to English for missing keys.
Read the Localization Document carefully to understand it.
You should not configure another language as the default because it will have missing keys and that will result in null reference exceptions when those keys are encountered. Only the English resource files are complete so that should always be the default.
You can force your site to use German in general with
<add key="UseCultureOverride" value="true" />
<add key="site1culture" value="de" />
but this will also result in all users having German used for labels and resources and editors. However for missing keys it will still fall back to English and not throw exceptions.
So supported scenarios are
1. Using Autodetection with fallback to English (the default) and then users will see the editors in their own languages
2. Forcing a specific culture as shown above but not using fallback so all users see labels and editors etc in in the specified language.
It would be theoretically possible to configure auto detection with fallback to German like this:
<globalization culture="auto:de-DE" uiCulture="auto:de-DE" requestEncoding="utf-8" responseEncoding="utf-8" fileEncoding="iso-8859-15" />
but in practice this will likely result in some null reference exceptions because the German resources have missing keys.
If you don't have the "auto" part it will never go by the browser language preference and if forced to a specific culture as shown with the site1culture setting above it will not either.
We don't have any resource files for de-CH, so it will use the Resource.de.resx files
Hope it helps,
Joe