How to localize opt-out link in mailing
Yes, it's a know limitation. The language used is the language of the cron job. If you manually launch the scheduled task with the proper language, you should be able to send in the proper language.

The way we intend to solve it is to add some option to tell in which language we want the mailing to be send - cf. https://issues.civicrm.org/jira/browse/CRM-16352