My kids, at public school, learn Mandarin, Japanese and Hawaiian. Not immersion. But they teach these languages at school. My kids, are pretty good at it. From Kindergarten they teach these classes.
We are not fluent in it at home. But my kids, know it.
Kids, up until 13 years old, (per National Geographic magazine) says that these are the ages which kids learn languages best. And, learning other languages, actually develops more synapses in the brain.
My kids are multi-lingual, per the languages they learn at school, and at home, my Husband is bi-lingual as well.
Kids learn it.
According to a study, which my Husband saw on the news, the top 2 languages per global usage, today, is: Chinese and French. Sorry but I forget what news channel this was. But of course, each study and lists, vary.
Just do a Google search.
And, there are the top languages per business usage, by native speakers by population, and by global usage or the most widely spoken language, etc.
So the variables, vary. Per the purpose.