On n'attrape pas les mouches avec du vinaigre

Literally: You don't catch flies with vinegar

Example: “Sois aimable avec le voisin si tu veux emprunter sa tondeuse ; on n'attrape pas les mouches avec du vinaigre.” — Be nice to the neighbor if you want to borrow his mower; you catch more flies with honey than with vinegar.

English equivalent: You catch more flies with honey than with vinegar

Origin: The full sense depends on the implied contrast with honey, which English states outright. French leaves the honey unsaid, which makes the vinegar do more work.