Correct command

CommandPing.java

public class CommandPing extends CommandBase {
    @Override
    public String getCommandName() {
        return "ping";
    }

    @Override
    public String getCommandUsage(ICommandSender sender) {
        return "commands.ping.usage";
    }

    @Override
    public void execute(MinecraftServer server, ICommandSender sender, String[] args) throws CommandException {
        if (args.length == 1) {
            sender.addChatMessage(new TextComponentTranslation("commands.ping.success"));
        } else {
            throw new WrongUsageException(this.getCommandUsage(sender));
        }
    }
}

en_us.lang

commands.ping.success=The server says pong
commands.ping.usage=/ping <something>

es_es.lang

commands.ping.success=El servidor dice pong
commands.ping.usage=/ping <algo>