Messages_en.properties

cmd.set.unknown_prop=''{0}'' is not a valid chest property!

SimpleLocalizedMessages.java

import org.bukkit.ChatColor;

import java.text.MessageFormat;
import java.util.Locale;
import java.util.ResourceBundle;

class SimpleLocalizedMessages {

    /*** USAGE ***/

    public static final MutableMessageSource MESSAGE_SOURCE = new MutableMessageSource(ResourceBundle.getBundle("Messages", Locale.ENGLISH)) {
        @Override
        public String getString(CharSequence key) {
            return bundle.getString(key.toString());
        }
    };

    public static void main(String[] args) {
        // Prints the default english message
        System.out.println(Messages.CMD_SET_INVALID_PROP.get("asdf"));
//        MESSAGE_SOURCE.bundle = ResourceBundle.getBundle("Messages", Locale.FRENCH);
//        // Prints the french message
//        System.out.println(Messages.CMD_SET_INVALID_PROP.get("asdf"));
    }

    public static final class Messages {
        public static final LocalizedMessage CMD_SET_INVALID_PROP = new LocalizedMessage(MESSAGE_SOURCE, "cmd.set.unknown_prop");
    }

    /*** LIBRARY ***/

    public interface MessageSource {
        String getString(CharSequence key);
    }

    public static abstract class MutableMessageSource implements MessageSource {
        protected ResourceBundle bundle;

        public MutableMessageSource(ResourceBundle bundle) {
            this.bundle = bundle;
        }
    }

    public static class LocalizedMessage {
        private final MessageSource messageSource;
        public final String key;

        public LocalizedMessage(MessageSource messageSource, String key) {
            this.messageSource = messageSource;
            this.key = key;
        }

        public String get(Object... args) {
            String message = ChatColor.translateAlternateColorCodes('&', messageSource.getString(key));
            return MessageFormat.format(message, args);
        }
    }
}