Language Fast

learn the grammatical structure then try to be exposed to the language every day, more input into your brain is the idea, and when you encounter words/slangs/phrases that you don't understand always look up what they mean every single time

Consistency is the key... you have to go at it every day or you will lose it.
I was getting semi decent at learning german and eventually lost it because i stopped my daily practice... same with programming languages.
every day... dedicate at least 1 to 4 hours and you will have made a big "leap" by next year.

the FASTEST way is to learn the basics with a book, and move to the country where that lang is common, it's not practical tho.

Try emerse yourself in a forum that speak that language?

You gotta go to the place mate and just immerse yourself. I did that with an abroad trip in Spain!

watch movies , go there , listen and speak with ppl in that language you wanna learn ...so 1-2 years and it will be better than my english haha

I think about this a lot, what are the basics? how do infants learn?

Cultural immersion is one of the best ways. You could learn from a book, but you would get a better understanding of the language and culture you are trying to learn if you apply it to your everyday life.

thanks for the precious information, I needed it to learn Spanish, I was thinking of moving to Barcelona soon

Duolingo is great to start, but then you will have to practice it by speaking to native speakers. That second one can be really tricky at times.

Practice by going to stores and trying to talk/ask about anything. Move on to talking with people.