Based on OP the number of English and Mandarin speakers is roughly the same, and considering written Chinese is mostly shared between languages that would mean that more people can currently read Chinese than English.
You have your qualms about Mandarin, but anyone can complain about a language they do not speak, and claim it is too difficult for this or that reason. Compared to English, Chinese grammar is very simple, the written language is compact, and there is little confusion about pronunciation. It seems as good as any (if not better than most) as an international language.
It seems you realize it as well, as it is hinted in your own posts; you will never convince everyone to pick your preferred language and convert to it. It is like trying to convert everyone to the same religion. You may think that your choice of [language/religion] is the best one, the one that makes the most sense, without realizing your own bias.
With that mentality I doubt that you speak a foreign language. If that's the case, of course you think that everything "important" is translated for you already - it's not going to show up on Google if you don't search it in its published language. Considering your tone I doubt I can convince you, so all I will say is that you don't know what you're missing.
That is not relevant to the post, which assumes that the reader is actually interested in learning a language. Many (I would say most) high school students are not, and don't invest more than the minimum amount of time required to pass their courses.
Yes, if you do not practice regularly (preferably daily), and with persistence, you will not reach a high level at a foreign language. Relatively speaking, maybe math is easier. But that's not much of an argument for math and against language learning, unless you have no actual interests and just want to check a box off a list.
To be fair, many (and maybe most) adults who claim to be interested in language learning also end up giving up because they are not willing to put in the effort. Too bad for them, and for you if you find it a "waste of time and money"; I find it far more rewarding than flaunting advanced math knowledge.