I am looking forward to having more ideas about the relationship between learning foreign languages, English in particular, and sustainable development.
I doubt the following is what you originally had in mind, but I can't resist pointing out that learning a language (or for that matter anything else) is an excellent example of moving toward sustainable development through the "dematerialization of consumption". Learning involves only a small amount of tangible resources, but it does require a lot of labor, both from the learner and the instructor. It therefore adds to GDP, but does not require steel, water, plastics and energy. It's often said that the world cannot continue to "grow" because of the physical limits of natural resources. But it can grow (I prefer to use the term develop) almost infinitely in areas that do not require tangible inputs--education, art, cultural expression, interpersonal relations. This view, I think, gets environmental advocates out of the supposed trap that rests on the view that they inevitably must seek a "no-growth" economy and, arguably, a stagnant society.
I particularly like Robert's answer since it involves learning not for a specific purpose--like Spanish for medical professionals--but learning for the sake of less tangible matters. I teach international students the majority of whom are from the Middle East and Asia; they are learning English to study at graduate level in the United States. But contained within that very specific goal are those that Robert speaks about--cultural expression and interpersonal relations. Once we learn even a portion of another language and have the opportunity to live and work among the speakers of that language, there is a shift in our attitudes about that culture, its people, etc. It is much harder to get into that mindset of "us" vs. "them" once we have lived among "them." If we eliminate some of that national competitiveness, then perhaps we can work on environmental issues with English as the language of business and commerce. No matter how good our English is, if we have not expanded our minds and cultures to be more understanding of one another, all the English in the world will not help us much.