Hi there Max,
Sorry to hear you are experiencing difficulties.
I’m not familiar with Ceceppa Multilingua but it’s quite possible our respective plugins are both trying to achieve some of their goals through the same WordPress mechanisms – but just aren’t meshing nicely.
Unfortunately in this instance I’m not sure how much we can do to assist – when plugins deliberately modify URLs dynamically (as it sounds like happens here when you switch between languages) that can be quite a tricky thing to debug – and as you can imagine, there are lots of multilingual plugins that all do things differently.
On this occasion it may be best to reach out to the author of Ceceppa Multilingua for further assistance as I’m not sure there will be too much we can do to assist on this occasion.