i placed the file "Telerik.Windows.Controls.resources.dll" I found in C:\Program Files (x86)\Progress\Telerik UI for WPF R1 2019\Binaries\WPF45\de in the folder "Libs/de" and added the following lines to my code:
Thread.CurrentThread.CurrentCulture = new CultureInfo("de");
Thread.CurrentThread.CurrentUICulture = new CultureInfo("de");
As you can see on screenshot "spellchecking" the Spellchecking-Dialog is not translated correctly. The button "close" is the only button that has the correct translation "Schließen".