what is so illegal about george orwells book animal farm that is said to be illegal in france ?

I think some governments might find the themes subversive. Animal farm is all about rebellion, communism and the rot of human nature (through animals) behind it. I'm not sure about France but certainly ultra socialist states like Cuba and China, might not like the messages behind it. The people in control like to think of their country is a "people's utopia" rather than a rotting autocratic tyranny.

Here is some info about specific locations this book was banned,

I found out that some French took exception to the name of Napoleon being used. In the first French version of Animal Farm, Napoleon is called César, the French form of Caesar. Allied forces found this entire book to be critical of the U.S.S.R., and therefore the text was considered to be too controversial to print during wartime. Publishers were reluctant to print the novel then. A play of Animal Farm was banned in Kenya in 1991, because it criticizes corrupt leaders. In 2002, the novel was banned in the schools of the United Arab Emirates, because it contained text or images that goes against Islamic values, most notably the occurrence of an anthropomorphic, talking pig.