In "Animal Farm", how does the original version of animalism become the slogan for legs good to legs bad?Do the animal rule with simple language?...

The original 7 commandments that are painted on a sign are too difficult for most of the animals to remember, most of them can't read.  When Snowball is beginning to organize the animals into committees and the sheep are only able to learn the single line, four legs bad, two legs bad, it becomes the motto of Animal Farm, replacing the seven original commandments which eventually are distorted to serve Napoleon's agenda.

 It symbolizes who the animals should trust and not trust.  Humans, who walk on two legs are to be feared and never trusted, four legs, animals are to be trusted. 

"The animals learn to read according to their limited abilities. The sheep learn a single maxim which embodies the essential principles of Animalism: “Four legs good, two legs bad.” Once they learn it, they say it for hours."

The story is not to be misinterpreted, the animals are representative of the actors in the Russian Revolution leading up to Communist rule and the birth of the Soviet Union.  So don't think of the animals as real animals, they are representing people, for example, Snowball and Napoleon are really Leon Trotsky and Josef Stalin.  So your question about language is not really relevant in the sense of real animals, who in real life can't talk.

Orwell called the story a fairy story or fairy tale, in such types of works of literature animals and magical characters take on human characteristics.