We have an umbraco installation with mixed languages in the backend. When I switch the language for my user to “English (United Kingdom)” or “English (USA)”, I still see some German words in the backend, for example. "Durchsuchen" instead of "Type to search ..." in the search field or "Entwickler" instead of "Developer", but not everywhere, as you can see in the following screenshot:

The default ui language is set to "EN"
<add key="umbracoDefaultUILanguage" value="en" />
Any tips or tricks on how to fix this?
Update
Looking through packages.config, I saw that these additional packages were installed:
nuget, , App_Data, bin obj , .