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 self.mailing_domain:
            domain = safe_eval(self.mailing_domain)
            res_ids = self.env[self.mailing_model].search(domain).ids
        else:
            res_ids = []
            domain = [('id', 'in', res_ids)]
        
        if self.contact_ab_pc < 100:
            contact_nbr = self.env[self.mailing_model].search_count(domain)
            topick = int(contact_nbr / 100.0 * self.contact_ab_pc)
            if self.mass_mailing_campaign_id and self.mass_mailing_campaign_id.unique_ab_testing:
                already_mailed = self.mass_mailing_campaign_id.get_recipients()[self.mass_mailing_campaign_id.id]
            else:
                already_mailed = set([])
            remaining = set(res_ids).difference(already_mailed)
            if topick > len(remaining):
                topick = len(remaining)
            res_ids = random.sample(remaining, topick)
        return res_ids
    def get_remaining_recipients(self):
        res_ids = self.get_recipients()
        already_mailed = self.env['mail.mail.statistics'].search_read([('model', '=', self.mailing_model),
                                                                     ('res_id', 'in', res_ids),
                                                                     ('mass_mailing_id', '=', self.id)], ['res_id'])
        already_mailed_res_ids = [record['res_id'] for record in already_mailed]
        return list(set(res_ids) - set(already_mailed_res_ids))
    def send_mail(self):