There is no French equivalent that I could find.

All there is, is the original Marquesaan (Polynesian) word “puaa ao” which can be translated into English as “long pig that speaks”.
You could translate that literally : “Le grand porc / cochon qui parle”.

I have never heard of a similar expression. To convey meaning, my best guess would be to translate it simply by “chair humaine” (literally “human flesh”). If you want to preserve style, I can think of anything better than @Kwaio.

Long Pig is a culinary term that refers to human beings.
The most accurate translation would be “Chair humaine”.

You could also use “Bidoche” or “Barbaque” if you want slang words about “Viande” (aka meat).

There are lots of expressions about Viande on cntrl.fr, but nothing about cannibalism.

From wikipedia about Cannibalism:

The friar Diego de Landa reported about Yucatán instances,[67]and
there have been similar reports by Purchas from Popayán, Colombia, and
from the Marquesas Islands of Polynesia, where human flesh was called
long pig[68].