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Rust.

The serious answer would be Japanese so I don't have to struggle with the kanji alphabet and their counting, having different words for numbers depending on what is being counted broke my brain.

I’m fond of Japanese, Chinese and Korean is nice too, Spanish is comforting, also French and Italian sound beautiful.

But to make the most of this magical opportunity, I’d pick snap my fingers to learn Chinese (gonna go with Mandarin) because it’s a daunting language to read, write, and pronounce. Would be nice to skip past the learning process lol.

As much as I am enjoying learning Finnish, I would love to be fluent. Niin suomea.

C++

I'd choose Japanese! I started studying it about two months ago and am really enjoying the process! But if there was a way to snap my fingers and be instantly fluent, well, I wouldn't say no lol

Or maybe I'd go for for JavaScript! Maybe an odd pick, but there's lots of cool things I'd like to make that require JavaScript knowledge way beyond my level, and a lot of times I don't even know what to google to just like, get started haha. And unlike Japanese the times I've sat down and tried learning from scratch I wasn't really enjoying it ;;

japanese

Spanish, because it's the second most spoken language in the U.S., and world-wide. Also, I live in New Mexico.

Second would be French. Because, I've always wanted to visit France, and I suspect that if I were perfectly fluent, then I'd be more accepted there, in spite of being American.

My first urge is to say Japanese, but thinking about it further, I may have more to gain if I learn "Classical Chinese" as the late Ming imperial scholars know it. It not only helps me with learning Japanese, but also Mandarin Chinese. It may also help me with some languages such as Korean and Vietnamese—but not to quite the same extent as Japanese and Mandarin Chinese though.

My other two choices are Latin, Sanskrit, and Classical Arabic, for similar reasons.

But Classical Chinese is it for me. Maybe I can do Kanbun within my lifetime through it and Japanese.

Vietnamese. There's a large Vietnamese population in the area I live, I frequent Vietnamese grocery stores, and I have always wanted to visit the country.

If, as others have said, programming languages count, I'd go for Cobol. As I understand it, lots of big banks still run on it, so there's good money to be made. It would probably help me to learn other programming languages too.

If it had to be a human language, I'd be tempted to go for Russian or Arabic because of the potential threats, so translation opportunities, or Welsh. I'm Welsh, but really struggle with learning Welsh, especially when compared to French and Spanish.

Language of love ( ͡° ͜ʖ ͡°)

This is probably not the answer that you are looking for but the programing language that would make me the most money via work. Or maybe the one that has the most longevity.

definitely c++ or Japanese

Arabic. Its akin to speaking Latin in Europe. There are regional differences but you can understand most people from a vast region and most immigrants Germany speak some form of it.

Chinese or Hindi, first because it can be useful and I have some understanding of English and Spanish already so they are out, second for personal reasons.

Probably none. The journey is half the fun with these things, though if I can finally get Chinese tones the offer is pretty tempting.

I think I would choose japanese just for anime.