But you almost certainly have gaps and misunderstandings in your native language too. I’ve learnt more than one language, almost entirely through immersion, and my grammar is pretty good. But I’d say if you want to do it well, you have to ask people questions. I was primarily talking to colleagues and friends while working overseas, and I’d say things wrong all the time, and get corrected, and ask questions constantly when I didn’t understand something. The real issue people have with immersion is that you have to be prepared to make a fool of yourself all day, every day, for quite a long time before you really start to get the hang of it.

The really significant upside that I think people tend to gloss over is that my pronunciation is pretty much perfect, because I’d learn to say things by just repeating what I’d heard. On the phone people can’t tell I’m not a native speaker.