You can change this from the menu: Settings > Technical > Actions > Report

To access this menu, your user need to be in the Technical Features group.

Search the correct report using invoicekeyword.

Then, change INV by FAC in the field Save as Attachment Prefix.

You can create a new module which adds the translate=True option to the column "attachment" of the ir.actions.report.xml object.

You need something like this in a new module:

class report_xml(osv.osv):
    _inherit = 'ir.actions.report.xml'

    _columns = {
    'attachment': fields.char('Save as Attachment Prefix', size=128, help='This is the filename of the attachment used to store the printing result. Keep empty to not save the printed reports. You can use a python expression with the object and time variables.', translate=True),
    }

report_xml()