I'm not fluent in Italian, which I usually notice when I can't come up with the word for, say, "doorknob", or when I have trouble with a sentence or find myself looking up words to make basic sense of a passage. Tonight I was looking at a recipe which called for cavatappi and I thought, "remove-corks" then "tire-bouchon" then "corkscrew" then "oh, corkscrew pasta". That's what not being fluent really means - even the stuff you know creates little eddies, increasing the impedence or Reynolds number of the language flowing through your brain.
For me, a former physicist and language enthusiast, this metaphor is very apt. There is a turbulence, a randomness, to the flow of ideas and language through the brain. And there are multiple opportunities along the way for linguistic eddies to form and impede or garble the output. More on that another time.