How to set up terms lists to consider a current term ?

Here is my situation:
I have many taxonomies for my custom post type “project”:
taxonomy “city” with terms like “Paris”, “New York”, etc. ;
and taxonomyB, taxonomyC, etc.

I have to implement “city” tax in a menu at top of the page (in header.php). Users can choose a city (or not, then it displays all cities’ projects).

So the url could be :
http://www.website.com/city/city-slug

If user choose one, I would like to set up the widget tax terms list to consider the selected city.

At this moment, it only works if I add “city” tax in the QMT widget, but it’s not what I want to do :/
Or maybe hide “city” tax in the widget ?

If anyone could help me ? 🙂

(Sorry for my bad english ^^)