Le "sot-l'y-laisse" is a small but delicious part of the chicken, it means "the fool leaves it there" in French. What is the English translation?

It's the oyster. A very tnder and deliciuos part of a fowl.

https://en.wikipedia.org/wiki/Oyster_%28fowl%29

Oysters are two small, round pieces of dark meat on the back of poultry near the thigh,[1] in the hollow on the dorsal side of the ilium bone. Some regard the "oyster meat" to be the most flavorful and tender part of the bird, while others dislike the taste and texture.

Compared to dark meat found in other parts of the bird, the oyster meat has a somewhat firm/taut texture which gives it a distinct mouth feel. It is also customary for the cook to be given first preference to the oyster meat.

In French, this part of the bird is called sot-l'y-laisse which translates, roughly, to "the fool leaves it there",[2] as unskilled carvers sometimes accidentally leave it on the skeleton

https://en.wikipedia.org/wiki/Oyster_%28fowl%29