The form is a "test" that my students have to fill - I send them the file and they have to rewrite and correct the text - so that's why i want to disable the "spell checking"
Here is an exemple of my document :
Thanking you in advance... merci beaucoup à l'avance
Spell checking is controlled by the PDF viewer installed on the user's machine. There is no way to configure it in the form itself.