[交流] 【Activity】What non-English language will be most useful for my child

What non-English language will be most useful for my child to know in 20-30 years?

I don't quite know the answer, but I would agree that it is Mandarin. Almost everyone who has answered this question so far -- and there are some great answers -- took "most useful" to mean "most economically advantageous."

Most useful ≠ most economically advantageous.

Now, I'm certainly not saying that money isn't useful. I'm just saying that your child may find another utility for language: travel.

One of the most important uses for a second language is the ability to travel more easily, more richly, and with a greater depth of appreciation than a traveler lacking language abilities. Let's reframe the question:

Most useful = Which language would be best for someone focused on traveling?

In order to answer that question fully, we would need to know where your child plans to travel, and the level of English proficiency in that area (i.e. an English speaker traveling to Sweden could learn Swedish prior to their trip, but I would argue that it would enhance the journey only minimally). However, if they will use their language abilities to help guide their travel, they should choose the language that opens up the most options.

No matter what, don't get so frustrated trying to choose the "perfect" language that you give up altogether - you really cannot make a wrong decision here.  Teaching your child any second language will be immensely useful to them.  Good luck, and I wish you and your child the best!