If I could only learn one additional language, and I wanted to travel the world, what language would serve me best other than English or Spanish?
It depends on your goals, and what cultures are you interested in.
I’m also fluent in Spanish and English (native Spanish though), and my third language of choice has always been between Chinese or Japanese. I like east asian culture, technology and many of their cultural exports. So it was an easy choice for me (easy the choice, languages… not so easy).