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. If Language property is not specified, English version will be displayed.
The code above will also localize the Asset Manager add-on.