Hi,
This looks like a problem with the Falang extension. I won't be able to give an answer as I don't know Falang well enough. I would recommend to contact the developers of Falang. They should be able to help.
Note that they just released a newsletter which asks to change the ordering of the Falang system plugin to fix an issue with the new version of Joomla regarding the switching of languages, so it might be related to that. Or it might be related to caching on the website, or something else. Again, it's best to ask them directly. They should know what to do.