For a standalone java application, the locale should be specified by the operating system and the default locale should reflect this.

I suspect that you are asking about a web server. For a web server, the desired locale is sent by the user in the header of the page. You read the header to figure this out. The header is "accept-lang".

You can then use that language in your response by getting the resource bundle for the requested locale and using that resource bundle to construct the responese:

ResourceBundle.getBundle(
  "com.mycompany.myapplication.mybundle",
  new Locale(language)
);