Hi,
There are some conditions that need to be met for JoomSEF to work correctly with multilanguage website:
1. Make sure that everything is enabled in JoomSEF's Control Panel in the SEF Status box
2. Make sure that System - Language Filter plugin is disabled
3. Make sure that System - ARTIO JoomSEF plugin is ordered as the first one of system plugins
4. Make sure that all your languages are set up correctly in Joomla's Language Manager on the Content tab
5. Make sure that "Enable JoomSEF language management" option is enabled in JoomSEF's Configuration on the Language tab
6. If you use FaLang or JoomFish, make sure that all your menu items, articles and other content are set to "All languages"