When using sh404sef plugin with multilanguage website you need to:
1. Go to Extensions > Plugins manager > and disable joomla language filter plugin.
2. Go to sh404sef configuration at “Languages” tab and enabled Multilanguage support.
3. Go to Extensions > Language manager > Installed – Site > and see how many languages you have in there, then go to Extensions > Language manager > Content > and make sure those languages are also PRESENT/PUBLISHED in there too.
4. Make sure you have a default home menu item for each of your website language, and also one default home menu item for “All” languages