I usually present my Diplome d'Ingenieur as a MSc in Engineering, on the basis that MEng programmes in the UK are shorter and considered slightly less thorough than BSc+MSc programmes. In fact, MEng is an undergraduate degree, whilst the French DI is closer to a French MSc and would be a postgraduate degree.
However, some universities also offer MSc in Engineering programmes in France, which are distinct from the DI. I personally do not care to make a distinction between University and Grandes Ecoles, but if the distinction is usually recognised in a specific industry (i.e. if the industry specifically wants certified engineers), one would be better off explicitly stating that their degree is a certified Diplome d'Ingenieur.