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.
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.
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.
Language of love ( ͡° ͜ʖ ͡°)
japanese
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.
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.
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.
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.
definitely c++ or 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.
C++
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.
As much as I am enjoying learning Finnish, I would love to be fluent. Niin suomea.