Splitting category display

OK, I’ve checked around all posts I could find relating to categories, but didn’t find any addressing specifically what I’m trying to do. I did try to fiddle with the php list_cats options, but I just can’t seem achieve what I want.
I want to have a bilingual blog, with some posts in French, and some in English.
The way I wanted to manage it is to have only two categories: French and English.
From there, I created sub-categories and sub-sub-categories.
Now, what I would like to display as navigation is two lists:
One of all the sub and sub-sub categories under French, with indents for the sub-sub.
One of all the sub and sub-sub categories under English, with indents for the sub-sub.
The name of the main 2 categories wouldn’t be listed as I would put titles for “French” and “English” over each list.
Can anyone give me a clue as to how to go about it or suggestions of other ways to go about it? With MT, I had simply hard-coded each link and could do a general update of additional categories through a module, but I’m clueless about how to manage it with WP.