I think Krashen's "Natural Approach" is the best way to learn. It focuses on consuming comprehensible input (CI) - listening/reading in the foreign language, and making sure you understand around 80%+. The idea would be to start with very easy stories/sentences, and slowly build that up as your vocabulary grows.
Pimsleur & FSI are good resources. Also, try to watch movies/shows that you already have seen in the target language instead (e.g. Friends, The Office, Simpsons). That way, you already have the context, and it will be easier to comprehend.
You need around 100-200hrs of CI to have a basic understanding of the language (maybe you can't speak, but you can understand basic interactions in the language). At around 400-600hrs, you'll be intermediate, and after around 800-1000hrs of CI, you'll be fluent.
Agreed. The FSI method is the best I’ve found, and Pimsleur is the best implementation of it. The biggest weakness IMO was that it was about listening and speaking and had only a minor reading component. The new software versions correct for that.
From there, you should be able to have some simple conversations and watch TV shows, at least with the foreign subtitles on. As a note, I found that (as in English) the subs might not match the spoken words, but I found that in some types of media (eg telenovellas) they match pretty well.
Are you young? Move there, get whatever work you can. You'll speak the language enough to get by within 3-4 months (depending on how much the language differs from your own). By 2 years, if you have an ear for accents and a little ability to mimic, you can be fluent enough to pass for native. It really helps to take language courses at the same time, or else you'll find your conversational skills to be excellent, but your reading and writing skills will suck.
Great, but if you learn somehing like german, you can't get a job in germany without passing tests, I think it is the same for english speaking countries.
You can easily get jobs in Germany without passing tests. Your options may merely be limited, but that's part of why I suggested this would be easier for a younger person. I got a job at UPS in Germany doing data entry and I didn't understand a stitch of German. This was many years ago, but the point is that there are many entry-level jobs where grasp of language is not so important, and EU laws are very amenable to migrant labor.
You may be picking grapes, or loading trash bins into the trash truck, but you'll probably be able to find some job where language skills are not required.
Incidentally, it's gotten much harder to do this as an American than when I did it, but as an EU member citizen it should still be easy. Plus, that's just the EU. OP didn't say what they wanted to learn. How easy this is varies from country to country, but the fact remains: it's still the easiest, fastest, and cheapest way to pick up a language.
If you're trying to learn a somewhat common second language where you live, and you want to supplement / reinforce other learning, I'd suggest changing how you watch television. Instead of watching, say, Stargate (bc someone mentioned it below) in English, watch it with the German soundtrack and English subtitles. You'll still be entertained, but you'll be learning as well.
You can also do something similar, usually with more language options, if you borrow movie DVDs from your library.
Do you know how, near the beginning of the Stargate movie, the leader of the Egyptian-like peoples was given a chocolate bar so they could extract the word for "good", and then the linguists from both cultures went around touching random objects naming each one? That's a good representation of how language learning would work if you were choosing something effective. Apps mainly don't 100% go that route and aren't taking the best modus operandi.
If you are trying to watch movies or series to understand a language i have a tip : don't select the best movie or series from the language you'll get too immersed and watch it in english dub to understand and enjoy it better (thats what i did).
Perhaps find somewhere local that offers part time courses, maybe a college etc. Or volunteer somewhere in an Italian community, for example, if you want to learn Italian.
I have no idea if the second idea would work but if you surround yourself with people from different cultures, maybe you might pick up words and basics.
Fastest?
To what level of competence?
Thorough / complete?
Accommodating the languages you already speak?
Grammar heavy? Vocabulary heavy? Balanced between grammar and vocabulary?
It might help to explain what your use of the language will be. Tourist? Academic? Moving there?
Some people are disciplined and can follow a self-made study course. Ie Duolingo, books, flash cards, etc.
Most people don't have the discipline or the interest, realistically. Learning a different language is a difficult thing.
In my opinion, the best way to learn a language is be forced to use it. So for example if you move to Germany, you're going to be forced to learn German so you're subconsciously and consciously paying much more attention.
You don't have to move anywhere necessarily though. For example I know Americans that speak fluent Portuguese. Why? They married a Brazilian.
I learned Spanish working with Cubans that didn't speak English. I already knew Portuguese so it was a lot easier for me, but I couldn't speak Spanish for most of my life. It was only when I had incentive to learn, did I learn.
Immersion, Girlfriends, Apps, Textbooks, Lectures are all good methods but the underlying problem is consistency. To learn a language you need to spend hundreds of hours studying so it won't happen in a day or a week. The best method is the one that you like and you are consistent in it.