Finds which i18n strings in a .properties file which are unused in the specified directory.
npm install -g find-properties-usage
fpu -F [path to the .properties file] -D [base dir of the project]
Additionally you can specify -p which is a comma-separated list of extra properties files to look in. You can also specify -i to ignore properties which are referenced in code but do not exist in the .propeties files.