I'm almost 50 in a highly technical field, but I can assure you that mastering 4 languages (Dutch, English, French, German - all reasonably fluently) is an enormous help. I have team members that are native French speakers with a limited knowledge of English. I have team members who are native German speakers and are quite fluent in English, but who still communicate faster in German. For me as the team leader it helps enormously that I can switch on the fly.
Much more important, however, even as a "technical manager" I constantly have to deal with suppliers & potential customers from all over the world. Being able to switch languages to their native one or at least to their second best one opens an enormous amount of doors. Germany is a particularly good example of this. Especially in southern Germany practical knowledge of English is limited - even amongst engineers. They are always very pleasantly surprised when they discover that a foreigner speaks German fluently enough to do business with them. And if "doing business" sounds not technical enough, the same applies to our field application engineers. As a worldwide company, we have field application engineers "everywhere", but we cannot afford to have them in every country. So we require them to be multilingual so they can cover a wider area, travel with ease, and deal with people who master English less than perfectly.
You say "how will German help me in Japan, China, Mexico, Spain, Canada, France, Norway, Iceland, Russia, Sweden or any other place?" And indeed, German will not help you in Japan. But it will help you in many European countries. French and Spanish will help you in a very large part of the world. Think of Africa & South America, for instance.
Finally, the whole point of the reported research is that having grown up in a multilingual environment helps in other ways than just knowing languages.I fully understand that this may be hard to believe for people who didn't have that luck - a bit like inhabitants of flatland can't imagine the third dimension. But that doesn't make it untrue.