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
addons/mass_mailing/models/mass_mailing.py
contacts_data = self.env['mail.mass_mailing.contact'].read_group([('list_id', 'in', self.ids), ('opt_out', '!=', True)], ['list_id'], ['list_id'])
        mapped_data = dict([(c['list_id'][0], c['list_id_count']) for c in contacts_data])
        for mailing_list in self:
            mailing_list.contact_nbr = mapped_data.get(mailing_list.id, 0)
class MassMailingContact(models.Model):
    """Model of a contact. This model is different from the partner model
    because it holds only some basic information: name, email. The purpose is to
    be able to deal with large contact list to email without bloating the partner
    base."""
    _name = 'mail.mass_mailing.contact'
    _inherit = 'mail.thread'
    _description = 'Mass Mailing Contact'
    _order = 'email'
    _rec_name = 'email'
    name = fields.Char()
    email = fields.Char(required=True)
    create_date = fields.Datetime(string='Create Date')
    list_id = fields.Many2one(
        'mail.mass_mailing.list', string='Mailing List',
        ondelete='cascade', required=True, default=lambda self: self.env['mail.mass_mailing.list'].search([], limit=1, order='id desc'))
    opt_out = fields.Boolean(string='Opt Out', help='The contact has chosen not to receive mails anymore from this list')
    unsubscription_date = fields.Datetime(string='Unsubscription Date')
    message_bounce = fields.Integer(string='Bounce', help='Counter of the number of bounced emails for this contact.')
    @api.model
    def create(self, vals):