Here's the Community Business College answer:
The answer to "What non-English language will be most useful for my child to know in 20-30 years?" depends on what your child's goals are. As an intellectual exercise, it is a good idea to pick the first language as one that is similar to the one they already know (e.g. Spanish or French). Then the next language can be something completely different (e.g. Mandarin, Hebrew, Japanese, Russian, etc.) that uses different characters and grammar rules.
It’s tempting to say that Mandarin will be a dominant language 20 years from now but I remember people saying that about Russian and the language spoken in India while I was growing up. I even had a teacher who pushed Esperanto. Needles to say, I don’t think we can call that one useful for everyday use.
If your child is going to learn it, you might want to start learning it as well. It's much easier to learn a new language if you have someone with whom to practice in an everyday setting.
As far as languages other than English for use in the adult world, it will make a difference where your child plans to live and work.
For example, our college set up inexpensive online language classes for our graduates with the primary purpose of helping English speakers start to learn Spanish. Having multiple languages improves their employability but for most of them, they plan to stay in California's Central Valley with a large population of Spanish speakers. More details are at http://www.cbcwebcollege.com/LearnALanguage.htm.
That said, we have had a couple of people who wanted to take Arabic and Japanese. There aren't as many speakers of those languages in this geographic area, but the students chose those languages for personal reasons. That is one of the best motivations for learning a new language. Motivation is an important factor when choosing a language to learn because it’s a skill that requires constant practice.
If you look at the globalization caused by the Internet, there will always be the need to be able to communicate in other languages, even though the translation software is getting better and better. The key here is to understand the nuance of a language, which means not just learning the mechanics of a language, but keeping the skill fresh with constant usage and practice.