I don't find in your code something for changing this, it is related to you or flutter ?
I set this in the code :
helpText: 'Sélectionnez un interval de dates',
fieldStartLabelText: 'Date de début',
fieldEndLabelText: 'Date de fin',
fieldStartHintText: 'Début',
fieldEndHintText: 'Fin',
(this code works for translating on the input side as requested in another issue)

Version : 1.0.2 (latest)