Administration and mechanical engineering are two totally different subjects. Which is it that you want to focus on?
Why do you want to apply mechanical engineering to developing countries, and what specifically do you mean by that? Mechanical engineering is a very broad field, typically only applicable to developing countries in the simplest of forms. What are you trying to accomplish?
Civil engineering is probably more applicable to your goals. Most people that want to “help” the developing world find that providing basic infrastructure – sewer and potable water systems – are the most needed.
Franky a degree in “Sustainable Development” sounds like a bunch of fluff. You probably don’t need a graduate degree to help the “developing world” – just fly to a country and start asking around… What exactly are your goals?