Naming Conventions:
<bundle>_<language_code>.properties

File Format:
InstallSet.installSetName=Custom
InstallSet.description=Choose this option to customize the features to be installed.

2. Create New Project,

3. Click on Locales and External Resource Bundle
Bundle Name : Test Bundle
Resource Bundle path : The custom locale properties file which you want to include.

4. Build and run the Project.

5. If you want to display or print the value of the key which is in the custom locale properties file, then you can add it in any of the panel,

Generally external resource bundle contains a properties file for each locale.