English: Under the seat, in front of your legs.
French: Sous le siège, devant les jambes.
Bags should be stowed fully under the seat in front of the passenger for take off and landing. However, you'll see that some passengers will inevitably leave bags between their legs or behind their legs for take off and landing. As you do your safety checks before take off or landing, you can remind these passengers to stow bags under the seat in front of them, emphasizing that the bag needs to go in front of their legs.
Devant=In front of
Les jambes (f, pl)=the legs
Pouvez-vous ranger votre sac sous le siège, devant les jambes, SVP?
Notice in English, we would say "in front of [your] legs", but in French, we say "devant [les] jambes", literally "in front of [the] legs.
Want to learn to speak the French you need to work as a flight attendant? Learn even more with Canadian French for Flight Attendants.