I’m not, generally, a big fan of the word ‘fluency’. It doesn’t do what people want it to do.
On the one hand, ‘fluency’ as meaning something like, ‘an ability to communicate at speed with accuracy’ is almost never what most non-linguistics people mean. On the other hand, ‘fluency’ as ‘a native-like ability to use the language without any real obstacles’ is what most people do mean, and this solidifies into a whole set of false notions…
I’d say that most people I talk to think that ‘fluency’ is a state that you achieve, the end of the language learning road. You go along as a beginner, intermediate, advanced learner, and then you arrive at ”fluency’ – where you understand everything, and speak like a native. And if you arrive at ‘fluency’, you also stay there – you can’t lose it, it’s locked in, and you can’t progress, because where would you progress to?
But the more you stare at ‘fluency’, the more it looks like a cardboard cut-out of the Eiffel Tower, and less like an Eiffel Tower. And then you get up close and walk around it. And realise it’s a 2d construct…
Because people do lose, or at least deteriorate, in their native languages (the ones they are fluent in), given the right (or wrong) set of conditions. A quick read-up on the research on language attrition makes it clear that even people’s L1s can decline, given lack of exposure and immersion in environments where L1 input is low or entirely absent.
Because people keep learning their native languages. On any given day I’m receiving hours and hours of English language input, and producing English language output. I learn new words. I encounter various structures, nuances, collocations, idioms, spellings, usages, and all this is continuing to shape my ‘English’. You never stop learning your native languages (provided you are exposed to input), because your mental representation of those languages continues to be shaped by that input.
Because there are things written in English that I simply cannot pick up and understand. Technical language, highly idiomatic language, emerging slang terms. These things may all be English, but without the requisite exposure, I can’t understand them. Here’s a fine example:
Besides its theoretico-didactic interest (it reveals the difference between Marx and Hegel), this representation has the following crucial theoretical advantage: it makes it possible to inscribe in the theoretical apparatus of its essential concepts what I have called their respective indices of effectivity. What does this mean? (Louis Althusser, “Ideology and Ideological State Apparatuses“)
What does this mean indeed. It takes a lot of mental parsing to process that sentence for me, and I do not possess the requisite background information to tell you what the respective indices of effectivity are.
This is why I shy away from talk of ‘fluency’, because it’s an impossible yardstick. There’s no final state of language attainment (except at death). There’s only progress, or attrition. And there’s only competencies – could I do this communicative task at this particular time (expression, interpretation, negotation of meaning) and with what degree of ease.