You can localize the Editor to be displayed in specific language by setting Language property:
<editor:wysiwygeditor
Runat="server"
scriptPath="scripts/"
Language="de-DE"
AssetManager="/assetmanager/assetmanager.aspx?c=de-DE"
AssetManagerWidth="570"
AssetManagerHeight="510"
Content="Hello World!"
ID="oEdit1" />
The current available values for Language property are: da-DK(Danish), nl-NL(Dutch), fi-FI(Finnish), fr-FR(French), de-DE(German), zh-CHS (Chinese Simplified), zh-CHT (Chinese Traditional), nn-NO(Norwegian), es-ES(Spanish), sv-SE(Swedish). it-IT(Italian). If Language property is not specified, English (en-US) version will be displayed.
The code above will also localize the Asset Manager add-on.