In Odoo 10 I had the same problem and solved it in a similiar fashion. I orientated myself by the function formatLang() of the misc.py file: https://github.com/odoo/odoo/blob/10.0/odoo/tools/misc.py.

Model:

class AccountInvoice(models.Model):
    _inherit = 'account.invoice'

    def _formatLang(self, value):
        lang = self.partner_id.lang
        lang_objs = self.env['res.lang'].search([('code', '=', lang)])
        if not lang_objs:
            lang_objs = self.env['res.lang'].search([], limit=1)
        lang_obj = lang_objs[0]

        res = lang_obj.format('%.' + str(2) + 'f', value, grouping=True, monetary=True)
        currency_obj = self.currency_id;

        if currency_obj and currency_obj.symbol:
            if currency_obj.position == 'after':
                res = '%s %s' % (res, currency_obj.symbol)
            elif currency_obj and currency_obj.position == 'before':
                res = '%s %s' % (currency_obj.symbol, res)
        return res

QWeb:

<span t-esc="o._formatLang(amount_by_group[1])"/>

Because there is NO solution about formatting numbers and no answer regarding wrapping RML reports, my solution is a function inside report py file which can be called from Qweb reports. You can select number of decimal points (default is two) and display currency symbol if you pass in the currency object. Function is a mix of code from ir_qweb.py (class FloatConverter and class MonetaryConverter). If someone has a kind of direct solution or can do better coding please....

class report_invoice(models.AbstractModel):

_name = 'report.account.report_invoice'

def convert_2float(self, value, precision=2, currency_obj=None, context=None):

    fmt = '%f' if precision is None else '%.{precision}f'

    lang_code = self.env.context.get('lang') or 'en_US'

    lang = self.pool['res.lang']

    formatted = lang.format(self.env.cr, self.env.uid, [lang_code], fmt.format(precision=precision), value, grouping=True,     monetary=True)

    if currency_obj:

    if currency_obj.position == 'before':

        formatted = currency_obj.symbol + ' ' + formatted

    else:

        formatted = formatted + ' ' + currency_obj.symbol

    return formatted

@api.multi

def render_html(self, data=None):

report_obj = self.env['report']

report = report_obj._get_report_from_name('account.report_invoice')

docargs = {

    'convert_2float': self.convert_2float,

     'doc_ids': self._ids,

    'doc_model': report.model,

    'docs': self,

    }

return report_obj.render('account.report_invoice', docargs)

And the call from Qweb:

<span t-esc="convert_2float((o.amount_total - o.residual),2,o.currency_id)"/>