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.
argentina doesn’t have a second official language but guarani is the closest i guess, specially since it is highly spoken in paraguay. i’ve met some people who speak it and in some provinces it’s common to use guarañol which mixes both (usually when you’re insulting someone you switch to guarani lol). so yeah it really depends mostly because depending on the province there is either one or other aboriginal language which is more spoken.
Portuñol is spoken near the border with Brazil and Uruguay, though most Brazilians can understand what Argentinians and Uruguayans say and vice versa
there’s this very cool album by uruguayan chito de mello where he sings in portuñol. https://www.youtube.com/watch?v=vj4ciV6byoY&list=PLlyEVWgddML9Y9f4i-jtF0nBvB9oBUnIC