A great discussion of the language learning process. On determining when a learner “knows” a language…

Lacking a clear line, language-learners have all manner of ad-hoc ways of describing when they finally got it. Some will say that the day they began dreaming in a language meant that they had it. But this is hazy; dreams, played in one’s own head, say little about real-world competence. Others recall the first conversation in which they did not have to struggle; this, at least, is a better rule of thumb. Others set the bar higher: only when you understand jokes do you really know the language.

The Economist: The Humble Linguist