Interesting topic. I've been wondering about this for a while myself. However my basic, or advanced, knowledge from some languages stem mostly from TV-shows. I'm a Swede, meaning english is a foreign languages for us in general. However I believe that video-games+tv-shows/movies and of course manga has made it into a 2nd language. Also spent around 3 months in England in total. However the small amount of korean and japanese that I've picked up is all due to either anime or dramas.

Idk how many different mangas, manhuas or manhwas that I've read but it's more than 3000. More to come!
(Number of H-mangas and one-shots: unknown and not included)

Over 180 anime shows watched and there's a vast sea of shows to go.

J/K-drama's 50+ shows watched.
It's Mr to you!

Yes, it helps a lot. Simple conversation yet useful.

I think reading manga is so useful to improve the language that you are trying to learn.

It does help because there may be words or phrases I'm not familiar with and thus I'd look them up and enrich my vocabulary

Nothing is Eternal but Eternity
Enjoy life as much as you can and do whatever you want, because it may be only one.

Yes, definitely that's why I read manga with English scans.

I learnt English solely thanks to reading manga and am now trying to do the same with Japanese. Wish me luck!

Having solid grammar knowledge can be good but you gotta do immersion to really learn a language and manga are perfect for that.

Well it did improve my command on English a little bit

I positively sucked at English (couldn't form a sentence for the life of me) until I started playing video games and later started reading manga online (read it in Swedish on the library before that)
Naturally it helps expands one's vocabulary and stuff, but mainly I think you need something to really catch your interest to make trigger it. I wasn't interested in English before video games, then I needed it to do something I enjoyed it and it just clicked. Grades went from a near F to B

Watching anime, movies and stuff and playing video games also helps. The most important thing is it should be something you have an interest, like Kain said. Your brain handles rest of it somehow.

Though you still can't speak well without practice. At least I can't.