Hi pilou,
As "Phiro" mentioned — you used an apostrophe (') inside your translation. JavaScript interprets it as the end of the string. To fix that, escape the apostrophe with a backslash (\).

correct:

'You have unsaved changes. Do you want to save before clearing the graph?' :
'Vous avez des modifications non enregistrées. Voulez-vous enregistrer avant d\'effacer le graphique?',