After enabling French and updating modules, the GTK client displays localized text in French.
But the main GUI of GTK client remains in English. Tooltips are also in English.
Tryton translation server (http://pootle.tryton.org) seems to offer module translation, not the main GTK interface. On Pootle, translations are marked 100% done in French at the time of writing.
How do I display GTK interface in French?