The problem is not (quite) with the concept of "fluency", it's that we we confuse it with being good at a language. Given your situation, I would just launch i

The problem is not (quite) with the concept of "fluency", it's that we we confuse it with being good at a language. Given your situation, I would just launch into a German explanation complete with the bit where I say I don't know the real German words. Then I will muck about with hand gestures and phrases like "smaller bits of would" etc until they get it. I would do all of this while screwing up my declensions and conjugations. And I would do most of the screwing up fluently. Less fluently if I notice the error.