From what I know, the more biotoy headcanons of fluffies just have fluffies speaking automatically at some stage, like this picture by AIDS here. This would make sense if fluffies were a type of replicant, aka what I call the biotoy canons, but I feel it would be out of place with the other canons that have more animalistic/natural approach to fluffies. Another simple explanation some people use is “programming”, but my own problem with this explanation is the fact that you can’t just program a language into DNA.
I do love the idea of adult fluffies taking an active role in teaching fluffspeak to their foals. One approach I had was that fluffspeak itself, due to some of its rules and regulations, could be considered a constructed language. Thus, one idea I had was that Fluffspeak originated from Hasbio itself, and early researchers taught fluffspeak to the early generations of fluffies. Then, whether it is the fabled PETA break-in, or domestics just ending up becoming feral, fluffspeak starts proliferating in the wild as ferals start eking out an existence independent of human contact. Also, and going back to the idea of adults teaching foals their language, I also have been writing about adult fluffies being used to teach foals in a daycare the fluffspeak language, along with the presence of a human teacher.