Some sort of Latin language would be useful (perhaps Latin itself). I have a basic grasp of Italian, but only just. If I could be fluent in that, I’d be able to also communicate in Spanish and Portuguese (I have many Portuguese friends) and perhaps even French. Romanian too.
Or I could go for something entirely different, such as Chinese. Might come in handy to be able to communicate with 25% of the world’s population overnight.
(btw I already speak fluent German and Greek, understant Dutch but cannot speak it, know some basic Czech, Russian and Polish, and also used to be fluent in Arabic, which I have completely forgotten now).