To externalize local messages, Java have the java.util.ResourceBundle class.
Create a properties file for the default locale. Then create a new properties for each locale with the suffix _<language>_<COUNTRY>, in the same way you initialize java.util.Locale.Locale(String, String).
Example
To load the localized text, use java.util.ResourceBundle.getString(String).
Example
messages.properties
messages_sv_SE.properties
And the Java code to load them.
Output