Do you have your resource properties file?
"Begin by creating a default properties file called MyConstants.properties in your GWT project. You can place the file anywhere in your module's source path, but the .properties file and corresponding interface must be in the same package. It's fine to place the file in the same package as your module's entry point class." http://code.google.com/webtoolkit/doc/latest/DevGuideI18nConstants.html#Constants
For example in com/google/gwt/i18n/client/Constants.properties containing
buttonText=Text of the button
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