As Admin:
- Set up 2 languages for the installation, e.g. English and French
- Admin language is English, Demo language is French
- Create a `res.partner.category` named 'Test English'
- Assign the category to a newly created partner
- Create a French translation for 'Test English', e.g. 'Test Francais'

As Demo:
- Create a Mass Mailing
- Select a custom domain: ('category_id', '=', 'Test Francais') => it
  should match one partner
- Save and validate

Now either:
- manually run the scheduled action "Process Mass Mailing Queue" as
  Admin
- wait for the cron to process the mass mailing

No mail will be sent since the domain will be evaluated in the Admin
language, i.e. English. Indeed, the domain
`('category_id', '=', 'Test Francais')` does not match any record in
English.

The solution/workaround is to use the language of the last user who wrote
on the mass mailing.

Closes #20906, Closes #15467, Closes #15445

opw-778922