Hello,
JoomSEF containes his own Language management, when Language Filter plugin in Joomla must be swiched off - multilingual support works in this way.
To use JoomSEF for multilingual site, please, follow these steps
1. System - Language Filter plugin must be disabled
2. System - ARTIO JoomSEF plugin must be ordered as the first one of system plugins
3. System - FaLang Database Driver must stay enabled
4. "Enable JoomSef language management" option in JoomSEF's Configuration on the Language tab must be set to Yes
5. Everything must be displayed as Enabled in JoomSEF's Control Panel in the SEF Status box
6. All your menus, categories and articles must have the Joomla Language option set to All
7. The languages you want to use must be set up in Extensions -> Language Manager -> Content and must be enabled
Make sure to delete existing SEF URLs in JoomSEF and clear all the Joomla and JoomSEF caches after checking the configuration.