USA: Spanish. I’m learning Spanish, and I think as a whole it’s like 18% of people in the USA but I feel like you’re likely to encounter some Spanish speakers and make friends even if not everyone speaks Spanish here.
Brazil: It looks like Spanish is also the second most spoken language with 4% or English with around 5%. I’m sure you’re more likely to find people on the Internet who speak it.
French: English, apparently a quarter of the population speak it??? Not sure how likely you are to find someone in the wild speaking it, but on the Internet, a lot of people.
My friend’s grandmother never really learned Castilian; she only spoke Catalan. I think she could understand it well enough to watch TV and get the gist of it. We tried to get her to speak it once; she didn’t get very far. I don’t know if she was literate, come to think of it.
But this was thirty of more years ago. Nowadays I suspect you’d be hard pressed to find a native Spaniard who can’t speak Castilian fluently.