These three books were translated by three different people :
Guide du Maître : Dominique Monrocq
Guide du joueur : Eric Holweck
Manuel des Monstres : François Marcela-Froideval & Michel Pagel
The translation quality is good for the "Guide du joueur" and the "Manuel des Monstres" and is not great for the Guide du Maître (see :
Grog critics (in french sorry)).
Though it is also important to point on the fact the "Guide du Maître" is by far the most difficult to translate because of its content.
On another note : François Marcela-Froideval is Casus Belli magazine founder and worked with Gary on MMII and Oriental adventures.
So the magazine was probably not gonna say it was a bad work
