I understand you're using the standard Joomla features. However, there's a Language Code plugin, a Language Filter plugin and a Language Switcher module. Each of those have their own configuration options. On top of that, there's the configuration you're using for your menus and the languages themselves. And, as you mentioned, there's the server environment you're running on.
We've tried to setup a failing language site, but have been unable too. Without understand what things you have enabled and how each of them are configured, it's difficult for us to narrow things down further or else we're just shooting in the dark.
Again, the best option would be to have a development site stripped of anything you wouldn't want us to see. That may also help eliminate any other extensions that could be also interfering with the language switching and JFBConnect functionality.
Beyond that, detailing out everything you can about each plugin, module and language setting you've configured would be a great help.
Thanks,
Alex