To fix the language problem, try copying all the files from the \Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\126.96.36.199__1f131a624888eeed\Resources\App_GlobalResources\ folder to the SharePoint web application's App_GlobalResources folder (e.g. \Inetpub\wwwroot\wss\VirtualDirectories\80\App_GlobalResources\).
The preview mode should be set like this:
However, when I tried to set this in the configuration file, I got an error. It seems there is a problem with setting the EditModes property from the config file. I have logged this and I expect that it will be fixed in the next editor release.
We are aware of the copy/paste problem from Visual Studio. The problem is that when you paste from VS, it automatically converts the clipboard content to HTML (to preserve the coloring/fonts) and this resulting HTML contains some overlapping tags. When these overlapping tags parsed, the editor automatically changes them so they are semantically correct. Unfortunately, this changes the layout of the pasted content (new lines are added). For now we have not been able to find a workaround for this problem. You will need to either use the "Paste as Plain text" editor tool to insert plain text or the "Format Code Block" dialog to paste the text and keep the coloring/format.
the Telerik team