Sascha,
We haven't done much testing with that plugin, but have added it to our task list to investigate for the upcoming 4.1 release. If there's something we can do to better utilize it's language switching abilities, we'll do it.
Unfortunately, beyond the above, I don't have a lot of ideas. The best suggestion I can give, which may or may not work, depending on how that plugin actually sets the language, is to make sure the JFBCSystem plugin is ordered -after- (higher number) the Language Filter plugin. However, I don't think this will correct the behavior because we generate the URL for the user to be redirected to in a different place, and so may not be seeing the language switch happening at all.
We'll see what we can do, and if we come up with a good solution before the release, will definitely let you know.
Thanks for pointing this out!
Alex