As a matter of policy, I do not translate 'département' unless a client specifically requests it, since the word is a specifically French administrative institution.

As a result, I think this is the least confusing way to express it, exactly paralleling the GB: County Council construction.



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Note added at 1 day43 mins (2006-12-04 20:49:28 GMT)
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Well, that's fine, if as a matter of course you translate 'département' by 'department', then naturally you would use 'departmental'; my own feeling is that informed readers generally are not stupid, and that it is no less meaningful to keep the specifically FR word 'département' to designate a specifically FR administrative entity than it is to translate it by 'department', which at best sounds odd, and at worst could be misread as referring to some other kind of organizational structure.

Of course, it all depends on your intended readership, their knowledge of French life and culture, and just how important it is for them to understand the niceties of FR local government organization. If a full and complete understanding is required, then you are probably looking at some kind of explanatory note anyway.