You can localize the Editor to be displayed in specific language by setting Language property:
<editor:wysiwygeditor
Runat="server"
scriptPath="scripts/"
Language="german"
AssetManager="/assetmanager/assetmanager.asp?lang=german"
AssetManagerWidth="600"
AssetManagerHeight="445"
Content="Hello World!"
ID="oEdit1" />
The current available values for Language property are: danish, dutch, finnish, french, german, schi (Chinese Simplified), tchi (Chinese Traditional), norwegian, spanish, swedish. italian. If Language property is not specified, English version will be displayed.
The code above will also localize the Asset Manager add-on.