For English it took me from 10 years old to 20 to be able to fool people in online conversations that I might be native. Now at 33 I am still daily picking up new English idioms and especially cultural background, but finally it is getting rare to encounter unknown words or phrases.

I still (rightly IMO) hesitate to say I can speak Japanese, even though I have lived here for 5 years and spent two of those solely studying the language. I encounter an unknown word in almost every sentence. Like someone here commented for German, having to explain so many words or think of synonyms is annoying enough that a conversation switch to English can happen if it is possible. Imagine talking to a speech recognition system that doesn't understand 10% of your sentences. It quickly makes you want to start typing instead.

I am still daily picking up new English idioms That's because we're making them up ;)

I once heard a definition that you are not really competent in a foreign language until you're able to solve crosswords in it. This principle is obviously tongue-in-cheek, but there's much truth to it in my opinion.

I can't do crosswords in English, my native language. I definitely can't breeze through classical texts. It's important to be realistic on the other end of the scale too.

Criteria like this are way too demanding, and it's one of the biggest problems in language learning that makes people feel worthless; having elitist standards that you "don't" speak a language unless you speak it like a university professor, regardless of the fact that you may not speak your mother tongue that way.

Well there's different types of crosswords. I guess they mean the ones that focus on the language itself (eg. idiomatic stuff) and not requiring some obscure knowledge

I personally like using the ability to understand stand-up comedy as the gold standard of fluency. I've lived 10 years in France now, and I was already capable of doing a job interview in French on arrival, until recently I only had French friends, only watched films / tv in French, read books in French 90% of the time, and I still struggle with comedy. Part of it is that they speak so fast, part of it is cultural references which I don't have, but a lot of it is that nuance is a major part of the comedian's trade, and I'm more or less nuance tone deaf, even after all these years of effort.

But... crosswords in Chinese? How'd you do that?

That's probably a very strict definition. I'd say that, if you're able to have a decent conversation without feeling frustrated all the time, you're doing good...

Let's say "able to study in a foreign university".

This, btw, automatically disqualifies all those tourists who think the could speak Hindi. Go ahead, visit a lecture in Benares Hindu University, or just a class in an Indian secondary school.

This is why I admire all those Asian and Indian students of MIT or Stanford.

I am biased because I work with tourists and have lived in Kathmandu for years, so I have seen innumerable cases of unsubstantiated self esteem or banal narcissistic personality disorder and tens of thousands of punks of any kind.)

Surely by that definition most native Hindi speakers aren't fluent in the language?

University study in a language requires strong literacy and mastering the higher registers of the languages.