Of course, before you can even start such a discussion you need to recognize that there’s a very wide range of “knowing” a language. When someone claims to “know” multiple languages–more than, say, ten–you have to take it with a grain of salt. Only a native speaker of each language, who spends a lengthy period of time with that person, in a variety of language-dependent and interactive contexts, can truly evaluate how much he or she really “knows” the language.
I work with professional interpreters on a weekly basis–interpreters of Arabic, Farsi, Haitian Creole, Spanish, Karen, among others. They’re all native speakers of those other languages; they aren’t bi-lingual, but their English is good enough to do their work professionally, and they do it well. There are ways, though, in which even they don’t “know” English, and I’m not referring to technical language, but with things that may seem to a native speaker very basic. Things like phrasal verbs. Take for example, these various meanings of the verb knock off.
1) The factory crew knocked off another thousand “Coach” handbags.
2) The salesman knocked off twenty dollars from the price.
3) They say the mob knocked off Jimmy Hoffa.
4) The men knocked off work and went home.
They might—though not always—be able to discern each of the various and distinct meanings from context, but even then, this is language they would not produce on their own. (In this case, I’ve asked them.) In other words, they only “know” this English receptively. They don’t “know” it proficiently, and will resort to other, non-phrasal verbs when they wish to communicate the same idea. And by saying, They say the mob killed Jimmy Hoffa, instead of # 3) above, they lose the particular connotation that the term carries.
On the other hand, most native speakers, of course, will not only clearly grasp the various meanings, but also will produce this language when the occasion merits it. Moreover, native speakers will “know” that one may say:
*They knocked off Jimmy Hoffy, * or They knocked Jimmy Hoffa off, as well as, They knocked him off.
But one may not say: *They knocked off him.
On the other hand, in what is essentially an arbitrary constraint, they know that while one may say, They knocked off work early,
one may not say: *They knocked work off early.
Native speakers also know that you can’t say, *The force of the wind knocked him off. (That is, they “know” that it requires a locative–e.g., knocked him off the porch, etc.–that this is not really a phrasal verb.)
The point of all this is that, without need for study, native speakers, purely by way of spending their formative years in the language community, can correctly and automatically produce this kind of complexity in language—the varied meanings of phrasal verbs (not just the example above) and how that governs their separability. The lexicon of phrasal verbs in English, which is essential, is too vast, with too many distinct meanings, presenting the same complexity as this one single example, to be something acquired through word-by-word study. By the same token, for a learner of English it would simply require too many hours looking at a dictionary, and very few language learners retain true proficiency with language in that manner anyway. A non-native speaker can choose to target some of them to study, but never the vast number native speakers "know.’ They really can only acquire this kind of proficiency, to any degree, through long-term engagement with the language, such as that which comes with long-term immersion–and usually much more than a “semester abroad.” Even then, because the “language organ” diminishes significantly after childhood, for adults we’re taking about a limited lexicon compared to that of a typical native speaker.
Obviously this kind of facility is not necessary, but it demonstrates how wide the range is of “knowing” a language–ways of knowing a language that is neither about studying nor “grammar.” It’s no different with other languages. Every language has something like this – levels of complexity (beyond grammar) which rarely can be acquired purely through study. So when someone says they “know,” say, 15 languages, you have to wonder just how well they know them, (for what purposes they can use those langauges effectively), and by which means they acquired this “knowledge.” How much time did they spend in those language communities?
The more I learn about native language facility in general, the less I’m inclined to say I “know” any language other than my own.