i18n: Fallback to fr.json

The i18n system in Svelte seems to allow one to define a fallback (error message: "[i18n]: No translation nor fallback found for 'header.signout'"). Let's set the fallback to French/fr.json.

added To do label

changed the description

assigned to @sbibauw

assigned to @bridubois and unassigned @sbibauw

added To review label and removed To do label

closed

changed milestone to %0.1: Basic test & chat ready

removed To review label