Kib,
I'm also using Ubuntu, and autocomplete is working ok for me (with one problem -- see below). It looks like the only difference between how you and I are using it is the language. I've got no idea how to get geany to use a different language -- I'm guessing there's a "use this locale" setting in Gnome somewhere, but I've never accessed it.
So, it seems like you might try switching your system to an English locale and see if the problem persists... though that's just grasping at straws.
The problem I'm having (please let me know if this should be in a new thread) is that I'm trying to create an autocomplete construct for "{{". It looks like this::
[HTML] {{={{ %cursor% }}
but it fails (that is, it doesn't do anything when I hit Tab except insert some spaces, as if "{{" was not an autocomplete construct).
---John