[TheOtherHobbes]

It depends how racist the country/situation is. Many countries will give English-speakers a break because many English speakers don't even try. So just being able to say something - anything - in the language counts as a plus. Anything after that is a bonus.

Even in English-speaking countries, you can screw up the grammar badly and still be comprehensible. US and British people may decide you're an idiot - and treat you accordingly - but they will mostly understand you.

So my personal ordering of importance is vocab, grammar, and finally accent/voice coaching. You will definitely need the latter for most languages, because even supposedly non-exotic ones like French require a novel set of mouth movements. Most lesson systems underemphasise this.

My working assumption as an adult is that it will take three months to learn basic conversation with full immersion and minimal distractions, and five years of constant practice to reach reasonable fluency in speaking, listening, and writing.

This is actually less time than it takes kids to learn a language, so the idea that kids are especially malleable or open is clearly wrong. Of course kids are learning language in general - and more - at the same time, so timings aren't absolutely comparable. But similarly adults are having to do a lot of distracting adult things at the same time, so it more or less evens out.