If you are in the US, Spanish is hands down the smartest language to learn, simply because it’s the one you will have the most opportunities for having authentic interactions in, and that means it’s the one that you are most likely to reach some meaningful level of fluency in it. Language is, above all, a tool for communication, and you learn best when you can actually use it as such.
Your second language is the hardest to learn, but once you get a language under your belt, it becomes much much easier to add to the collection. So choose an easy-ish one that you will have ample opportunities to use, and you are golden.
My next focus would be on regions where:
- You are likely to work
- English is not widely spoken
and languages that:
- You have some intrinsic interest in
- You are likely to have opportunities to use
Where you work matters a lot if you are considering your language as a financial investment. In the US, service-oriented job soften provide an automatic pay bump for Spanish speakers. If you work in International Development, French is the most marketable thing you can know because there is so much work in and with West Africa. Arabic was briefly a money train simply because the US had a very small number of Arabic speakers who were qualified to get security clearances. That has since dried up as the nation’s Arabic departments got swinging. If you do high-end manufacturing, German may be very helpful. But there is no language that is inherently worthless or inherently useful. It’s all about what you do.
The next consideration is how likely you are to actually need that language. Since Chinese is not widely spoken internationally and China doesn’t do a ton of work with other Mandarin-speaking nations, English has become the default international business language. In China, English is widely spoken among educated people. While it is nowhere near universal, all Chinese students take English (of admittedly variable instructional quality) from primary school through university. A common complain among expats is that they have a hard time practicing their Chinese, because everyone else wants to practice their English with them. In China, nobody expects you to learn Chinese, and there are even things like English-speaking ticket windows at the train stations. And so while China is big and increasingly important, Chinese is not particularly helpful to doing business in China.
In, say, Francophone Africa or Latin America, English is not widely spoken, as French or Spanish take the place of the “international language” and the business culture is aligned to French or Spanish speaking nations. You are much more likely to find yourself in business situations where nobody speaks English.
Finally, learning a language is tough, and few people have the fortitude to learn it without some deep interest in it. Studying a language is great for your cognitive development, but it’s not practically useful. Only actually speaking and understanding the language is useful. So the best language to learn is the one you are likely to stick with long enough to learn to speak.