I love that topic! I'm a big languages and linguistics nerd, but also a big advocate for people to learn other languages, especially in English-speaking countries where being bilingual seems to be more rare. It changes your brain so much, but also offers a kind of openmindedness in my opinion that doesn't come with speaking in only one language your whole life.
Whatever motivates you in life can be a way to learn a language!
I have been very
lucky myself, being born in a family of immigrants in a country that has 3 official languages itself, and with a mother who made me take languages lessons every Saturday morning. My favourite subject at school was Latin by far and, I must admit, I was the best at it. :p I cannot talk enough about all the benefits of learning a dead language, I want them back in the curriculums of every school in the world.

My love of languages in general came much later though, at university, when I had enough of the science.
What languages do you speak?
I fluently speak French, English and Polish. I have a very good level in Dutch and Norwegian. I have also learned German and Swedish before but haven't practiced in years.
What tips do you have for learning a new language?
Rewatch series you love in the language you learn, only with subtitles in that language. Research series originally in it too, as well as artists who sing in your language and keep listening and singing.
Reread books you love, find youtubers who speak it, anything else you're passionate about... Put your phone and your computer in that language. When you're out and about, observe and wonder how to say something in it, even just saying it in your head. Research words you want to use.
My last tip is, sadly, some self-discipline. ^^ For me, that means doing all that, and getting a textbook and studying my grammar and vocabulary lists.
What is the most practical language to learn?
Honestly, I don't find that learning a "common" language motivates people. If you have a special connection to a language or a country, that will be the most practical for you to learn. In my case, my husband has Norwegian family and I love Finnish culture, so these are the languages I want to put effort into. I have no interest in Spanish or Italian for example, other than my love for all languages. ^^