This is a purely French invention; I've never heard an Anglophone person say it! Firstly, there's already a meal that takes place between lunch and supper. It's called dinner - supper being the light meal taken much later in the evening, e.g. after returning from the theatre or a party. 'Slunch' also sounds very much like the meal known in Britain as 'high tea', which is a kind of buffet affair eaten around 5pm involving sandwiches, cold meats and cheeses, salads, cakes and scones, etc.
Unfortunate, too, that the word 'slunch' sounds to Anglophone ears very unpleasant indeed. It's close to 'slurry' (la boue), and generally, words beginning 'sl' are negative: slimy, slovenly, slippery, sly, etc. The worst thing is that 'slunch' already exists as a slang term: it's a contraction of 'sex' and 'lunch' i.e. when someone has an affair during their lunch break instead of using it to eat; it's also a term of insult for women, something like b*tch or sl*t. So... bad idea all round!