I think we should all learn languages - and that it's one of the things that can bring you great value to your life.
You would benefit from learning a language because first, you open up an entire world to access. So this means things like culture, comics, art, shows, and so on. So you get richer and more diverse sources of content - which sounds great!
Second, it's pretty useful for work. I think that languages are probably one of the most exclusive and valuable skills you can develop. Not many people put effort into this so you'll probably have a lot more access and be a more competitive candidate. And of course the main attraction is that you would get to choose to work in such places that speak said language.
Third, it expands your mind and philosophy. Learning a new language also helps you learn a new culture and all of its implicit codes and rules. When you adopt this specific cultural mindset, your lens of the world really does change. You realise that there are many assumptions or values specific to your own society. And I think it's a travesty to live in a closed off mindset for your entire life. Being able to understand or at least empathise with different cultural mindsets is really a paradigm shift.
So how should we learn languages?
Well the best way would probably be just to actually live among native speakers. And if you're forced to survive you'd probably pick it up in no time, like be intermediate in a few months at least.
Working downwards from this idealised state, perhaps the next best thing is immersion through content. So like watching shows without subtitles, working on basic grammar to get by, and going through the most frequently used words.
A key factor is definitely personal interest. Without that, whichever language you choose to pick up won't be so attractive. While it's good to have SMART goals, language acquisition is also a never ending state. As in, even if you attain fluency, there will still be so much to learn about a language and how to manipulate it.
So just like with life in general, language acquisition should ideally be enjoyed right now, because if you focus too much on just the outcome, you might be very unhappy for the rest of the time you spend chasing that goal.
Personally, I think the two main 'strategic' languages are English and Mandarin. Both are ideal because they encompass the largest swathe of people in the world. Both languages can also be used to branch out to other similar languages in the family. For example the Romantic languages from English, and Korean or Japanese from Chinese.
After that, I think personal interest should be even more heavily weighted in choosing any future languages. Since there are diminishing returns in terms of the pool of native speakers, you might as well choose something you're interested in. For me, I would be looking to Korean or Japanese, but only because of my interest in reading their comics faster and I'm so done with waiting for translations.
All in all, it must be so boring to be monolingual. Your world view is smaller, you enjoy less content, and you meet less people. Learn a language because it's fun, it'll help you in life, and so that you can have richer experiences. And let personal interest guide you, so that you can maintain your motivation and that you actually have a good time while learning the language - since that's literally what you will be doing all the way until you hit 'fluency'.