We can add a resourcebundle property file to our application to support internationalization (i18n). The file contains key-value pairs where the value is a localized value per supported language or locale. In IntelliJ IDEA we can easily see which keys are not yet translated. We open a resourcebundle property file and click on the
ResourceBundle tab at the bottom of the editor. We get a list of all available keys on the left and on the right a text area per supported language with the translated values. If a key is not translated for all supported languages it will be colored in red. We only have to select the keys in red and fill in the values on the right for the given languages.
Sample with IntelliJ IDEA 13.1.1