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
"""Return the recipients of a mailing campaign. This is based on the statistics
        build for each mailing. """
        res = dict.fromkeys(self.ids, {})
        for campaign in self:
            domain = [('mass_mailing_campaign_id', '=', campaign.id)]
            if model:
                domain += [('model', '=', model)]
            res[campaign.id] = set(self.env['mail.mail.statistics'].search(domain).mapped('res_id'))
        return res
    @api.model
    def read_group(self, domain, fields, groupby, offset=0, limit=None, orderby=False, lazy=True):
        """ Override read_group to always display all states. """
        if groupby and groupby[0] == "stage_id":
            
            states_read = self.env['mail.mass_mailing.stage'].search_read([], ['name'])
            states = [(state['id'], state['name']) for state in states_read]
            read_group_all_states = [{
                '__context': {'group_by': groupby[1:]},
                '__domain': domain + [('stage_id', '=', state_value)],
                'stage_id': state_value,
                'state_count': 0,
            } for state_value, state_name in states]
            
            read_group_res = super(MassMailingCampaign, self).read_group(domain, fields, groupby, offset=offset, limit=limit, orderby=orderby)
            
            result = []
            for state_value, state_name in states:
                res = filter(lambda x: x['stage_id'] == (state_value, state_name), read_group_res)