Direction Départementale des Territoires

10:43 Sep 29, 2016 

http://www.developpement-durable.gouv.fr/Les-DDT-directions-...
Wikipedia os often a great starting point. The in-text references or those at the bottom of the page are usually worth following up. I have not looked at the Wikipedia page here. As it is an official structure, there is an official page, or several; I have provided one to start the ball rolling.

This ought not to be translated as it is a proper noun.
But as a footnote, or after having used the propoer noun, in French (generally in italics where a foreign word is used), then you could use the French abbreviation and/or an "Englishified" version. If you do go with a translated version, then you need to keep very close to the original, at the risk of it not sounding that natural anyway. "Departmental Territorial Directorate" but that is not accurate as it does underline the fact the the structure is départemental, at a departmental level in the FR sense. "Territorial" as an adj. is confusing "for (the) Territories" is not helpful either in EN.

Try the conventional approach of using the French in italics w/ a footnote. Note that "département" and "région" are anything but synonymous in France! ;-

10:16 Sep 29, 2016 

My immediate reaction is that 'regional' is misleading. 'Régions' and 'départements' are both French government entities, so I would not use 'regional' in your answer as this would not be appropriate. Not my area of expertise so I cannot offer you an alternative, just this comment, sorry!