I use GWT i18n Messages to translate messages:
@DefaultLocale(\"en\") public interface Messages extends com.google.gwt.i18n.client.Messages { @DefaultMessag
GWT is always UTF8. So the easiest way to solve your problem is, to set the encoding of your workspace to utf-8.
Or select your Messages_de.properties, right mouse button and select properties. Use Resources to set the encoding of your property file to utf-8.