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
if hasattr(model, '_mail_mass_mailing') and getattr(model, '_mail_mass_mailing'):
                if getattr(model, 'message_mass_mailing_enabled'):
                    res.append((model._name, model.message_mass_mailing_enabled()))
                else:
                    res.append((model._name, model._mail_mass_mailing))
        res.append(('mail.mass_mailing.contact', _('Mailing List')))
        return res
    
    _mailing_model = lambda self: self._get_mailing_model()
    active = fields.Boolean(default=True)
    email_from = fields.Char(string='From', required=True,
        default=lambda self: self.env['mail.message']._get_default_from())
    create_date = fields.Datetime(string='Creation Date')
    sent_date = fields.Datetime(string='Sent Date', oldname='date', copy=False)
    schedule_date = fields.Datetime(string='Schedule in the Future')
    body_html = fields.Html(string='Body', sanitize_attributes=False)
    attachment_ids = fields.Many2many('ir.attachment', 'mass_mailing_ir_attachments_rel',
        'mass_mailing_id', 'attachment_id', string='Attachments')
    keep_archives = fields.Boolean(string='Keep Archives')
    mass_mailing_campaign_id = fields.Many2one('mail.mass_mailing.campaign', string='Mass Mailing Campaign')
    campaign_id = fields.Many2one('utm.campaign', string='Campaign',
                                  help="This name helps you tracking your different campaign efforts, e.g. Fall_Drive, Christmas_Special")
    source_id = fields.Many2one('utm.source', string='Subject', required=True, ondelete='cascade',
                                help="This is the link source, e.g. Search Engine, another domain, or name of email list")
    medium_id = fields.Many2one('utm.medium', string='Medium',
                                help="This is the delivery method, e.g. Postcard, Email, or Banner Ad", default=lambda self: self.env.ref('utm.utm_medium_email'))
    clicks_ratio = fields.Integer(compute="_compute_clicks_ratio", string="Number of Clicks")
    state = fields.Selection([('draft', 'Draft'), ('in_queue', 'In Queue'), ('sending', 'Sending'), ('done', 'Sent')],
        string='Status', required=True, copy=False, default='draft')
    color = fields.Integer(related='mass_mailing_campaign_id.color', string='Color Index')