English: Can you straighten your seat back?
French: Pouvez-vous redresser votre dossier?
Most airplane seat backs recline for passenger comfort, but they need to be brought back up to a vertical position for in preparation for takeoff and landing.
Redresser=straighten/put back upright
Dossier (m)=seat back
You may also hear "dossier du siège", literally "seat back of the seat".
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