Impacted versions: 9, 10

Steps to reproduce:

1. Install mass_mailing.
2. Install an additional language (i.e. Spanish).
3. Log in as demo user.
4. Create a new partner, set its country to be "Spain".
4. Change demo user's lang to Spanish.
5. Create a new mass mailing.
5. Choose customer recipients.
5. Select those customers whose country is "España".
4. Design the email.
3. Send it.

Current behavior:
The demo user sees 1 record selected, the admin user sees 0 records. This happens because admin's lang is English and no countries are found if you search for "España" under that context.
Since the queue is sent under the admin user context, no mails are sent.
Demo user scratches his head (in case he ever notices no mails were sent).

Expected behavior:
Queue should be run under the same language as the one in the context when the user pressed the "send" button, to avoid losing records when some domain is applied over translated fields.

@Tecnativa
if 'opt_out' in vals:
            vals['unsubscription_date'] = vals['opt_out'] and fields.Datetime.now()
        return super(MassMailingContact, self).create(vals)
    @api.multi
    def write(self, vals):
        if 'opt_out' in vals:
            vals['unsubscription_date'] = vals['opt_out'] and fields.Datetime.now()
        return super(MassMailingContact, self).write(vals)
    def get_name_email(self, name):
        name, email = self.env['res.partner']._parse_partner_name(name)
        if name and not email:
            email = name
        if email and not name:
            name = email
        return name, email
    @api.model
    def name_create(self, name):
        name, email = self.get_name_email(name)
        contact = self.create({'name': name, 'email': email})
        return contact.name_get()[0]
    @api.model
    def add_to_list(self, name, list_id):
        name, email = self.get_name_email(name)
        contact = self.create({'name': name, 'email': email, 'list_id': list_id})
        return contact.name_get()[0]
    @api.multi
    def message_get_default_recipients(self):
        return dict((record.id, {'partner_ids': [], 'email_to': record.email, 'email_cc': False}) for record in self)
class MassMailingStage(models.Model):