[FIX] improved date format in fiche de paie report
Merge pull request #572 from odoo-dev/7.0-opw-609069-fka

[IMP] l10n_fr_hr_payroll: make sure the date format of Fiche de paie report is in french

report_sxw: make sure we have a fallback on en_US if trying to load an format in unknown language