I would strongly disagree with "It’s like feeling that unexpectedly, you have a button in your brain. When you push it you can get thoughts straight to your target language." being the level of "at least being able to interact with locals".
In my personal experience, that's a point well beyond that. I was speaking fluid english for considerable time before being able to think in english on a whim. I can (usually) get my point across in german and russian, but thinking in those languages? No. I'm still translating to and fro in my brain, still formulating what I want to say in a language I feel more confident with.
From my experience, it takes total immersion in the language to become fluent. I spent two years in Brazil and it took 6-8 months before I reached the point where I began to think in Portuguese. It's a pretty amazing thing though, once you reach that point. You're thinking and even dreaming in another language. I still have dreams in Portuguese even though it's been 15 years since I returned to the US.
In my personal experience, that's a point well beyond that. I was speaking fluid english for considerable time before being able to think in english on a whim. I can (usually) get my point across in german and russian, but thinking in those languages? No. I'm still translating to and fro in my brain, still formulating what I want to say in a language I feel more confident with.