// the routes file
GET /i18n controllers.JavaScriptController.i18n()

// the controller
public class JavaScriptController extends Controller {
    public static Result i18n(){
        Lang l = request().acceptLanguages().get(0);
        String properties = "";
        // you can also use commons-io
        try(InputStream in = Thread.currentThread().getContextClassLoader().getResourceAsStream("messages." + l.code())){
            properties = new java.util.Scanner(in).useDelimiter("\\A").next();
        } catch (NoSuchElementException | IOException e) {
            Logger.error("Failed to read messages file");
        }
        return ok(properties).as("text/plain");
    }
}