We do not use any cron user since Simplify cron (32265209fd9e) · Commits · Tryton / Tryton · GitLab and even at that time we did not use the language of this user.
This is stored in the database and can no more be changed once the database has been setup. But it is actually the default language that the cron task will be using.
Indeed this is what you must do.
Usually for such task we set the language for the receipts.
Indeed you can use trytond.report.get_email so it creates for you the email message with the content from the HTML report translated for each language with the proper Content-Language parameters.