Thanks to Pete, Warren and Ben !!
My new script after all their informations :
put "defaults read NSGlobalDomain" into vxLangage
get shell (vxLangage)
get line lineOffset("AppleLocale",it) of it
put false into vxFrançais ; if "fr" is in it then put true into vxFrançais
get shell (defaults read NSGlobalDomain) don't work... !!?