I work as an interpreter and the two languages I speak other than English were learned in two very different ways. If we include English, then there were three ways. I won’t extol on the many different theories of language acquisition as the majority of them have already been mentioned and the truth is that these concepts are esoterically masturbatory because not every human who is able to use language is a linguist. In other words, I do not need to know the molecular arrangement of the silica of which the chip that is the brain of my computer is composed in order to use my Mac.
I learned English organically in the way that all children exposed to language take in the code that is used in their little slice of the human experience. The avenue for language acquisition is open (years 3ish through 7ish) and the data is written in that area of the human hard drive designed for it. The language becomes part of the OS used by my brain and I do not have to think about the words that frame the thoughts they represent. The words just come. Automatically. I cannot really explain how it happened, because like anyone else, I was just a child and I was not trained in the language if linguistics. It was not like I was thinking, “Oh, this verb is transitive and the prepositions that accompany the verb are this, and this, and this. The nouns connected with those prepositions will be in the dative case in these situations and under a few special circumstances, the case will in fact be the genitive, and there are at least two orphan phrases using the verb where the instrumental is used.”
I learned Russian at the hands of Uncle Sam at The Presidio of Monterey Defense Language Institute, Monterey California, in the late 80’s. It was grueling. Huge lists of words had to be consumed daily and it was only after seven or eight months of attending six hours of class, five days a week, nothing but Russian, did I begin to dream in Russian and start having difficulty writing with Latin script. At the end of the year long course, I found it next to impossible to write a cheque without having the young lady working the cash register say to me, “English, sweetie. English.” It was a difficult manner in which to learn a language and the only saving grace was that we were, all of us, learning the same language at the same time; thus, we spoke Russian amongst ourselves and created a little Moscow enclave.
Though I am Puerto Rican by birth, I was not raised speaking Spanish. I understood it with a fair degree of fluency, but the understanding was passive, not active. I could take in the information and relate it to what it meant in English, but I was unable to produce it with any amount of fluency. This is a common phenomenon with second generation immigrants. Difficult to explain. I moved to Puerto Rico five years ago and was tossed into the deep end, so to speak. It was a sink or swim situation. Linguists will argue that after about age seven, the route for language acquisition is closed and no amount of force will reopen it. I disagree in the extreme. I find it amusing that most people who study applied linguistics are themselves rarely polyglots. In fact, they usually rankle when they mention being linguists and the response they get is, “Really? Wow. How many languages do you speak?” Anyway… After about three years, I noticed that my Spanish was coming with fluidity. I noticed that I was not translating in my mind from Code X to Code Y in order to speak it. Each code was coming from the same nebulous area I call Concept. Now I speak Spanish without thinking about it. The words just come. I used to say, “In English I say what I think. In Spanish I have to think about what I’m going to say.” That is no longer the case.
But none of this waxing answers the OP’s question. My answer to the actual question is: immersion therapy. Nothing beats the exclusion of other codes, and the inclusion of need-based use to hammer a language into the right place on the old hard drive.