I’ve learned via experimentation that the best approach is to be incredibly awful- in fact, just above the point where they can’t you’re speaking French at all. This will cause them to appreciate your efforts, while simultaneously removing any risk that they speak French to you.
(the worst foreign language experience I ever have had, came when by some natural catastrophe I expressed myself in French succesfully well to earn a response in French, which I obligingly fumbled with sufficient skill enough to convince the barchap that I was German, a language in which I normally consider myself incompetent, but apparently competent enough that on this occasion I appeared Spanish to the confused Italian.)
The best case scenario here is that they think you’re retarded, and as a result are extra helpful, and possibly give you sweets.