If you're fluent in Spanish you can probably bullshit your way through comprehending most romantic languages so French and Portuguese (the other big colonial languages) are probably out.
Maybe Arabic for the fact that while it isn't a dominant language in most countries there are fluent communities in all sorts of parts of the world.
Alternatively, Hindi/Mandarin for the sheer number of people it'd let you communicate with.
Plus fluent in Spanish opens up travel to pretty much all of South America, Mexico, and Spain. There are differences in dialect, and some South American countries speak Portuguese, but you'll still be able to communicate.
Fluency in spanish doesn't help with french in my experience. While both might be romance languages, french pronounciation makes it so that in conversation words are really, really hard to translate without prior knowledge on some words, at least for me. My first language is spanish, for reference. Portuguese and italian are a 50/50 depending on the accent.