change the value of field json_value in ir.default model where the record for default user language  is defined ...  getting it changed from "en_US" to "your_lang_code" through the shell command can be done by typing:
self.env['ir.default'].search([('json_value','=','"en_US"')]).json_value = ' "your_lang_code" '
self.env.cr.commit()

Thank you for your answer, Theo!

I have set the auto_redirect_lang flag to false, both in the shell, as well as in the DB. It seems, however, to have no effect. The website still redirects to the browser's preference, regardless of it.

I have a standard demo instance, no extra modules or modifications.

Does this work for you?

Try this switch in a shell (assuming your website ID is 1):

$ self.env['website'].browse(1).write({'auto_redirect_lang': False})
$ self.env.cr.commit()