What does Isobel say to Katherine in French as she picks up the wine bottle The Vampire Diaries episode Know Thy Enemy?

Isobel doesn't actually say anything to Katherine in French in that scene. The line you're likely thinking of is when Isobel tells Katherine to "Drink up" in French, but she doesn't say it while picking up the wine bottle.

The scene you're referring to is from The Vampire Diaries episode "Know Thy Enemy" (Season 1, Episode 9). It occurs after Isobel has tricked Katherine into drinking a serum to prevent her from turning into a vampire, but which actually contains a powerful toxin.

Isobel then tells Katherine, in French, to "Bois, sois sage" ("Drink, be good"). The scene then cuts away as Katherine takes a sip of the poisoned wine.