I released another simple module on drupal.org the other day called Internationalization SSO. If you determine the site language from the domain used to access it, and you use different top-level domains (e.g. example.com, example.es, example.fr), this module makes life a little easier for you by logging users into all of the language sites when they log into any of them.
Special thanks to Project Ricochet for sponsoring the work.