Prominent languages can be thought of as natural monopolies because of their network effects. The high cost for a second language to become widely used incentivizes language learning really in only one direction. This of course is naturally understood because of the high popularity of english learning as a second language globally.
An article published by the Canadian economics association (http://www.jstor.org.libproxy.temple.edu/stable/135911?seq=1) discusses the policy implications that should be drawn from this as a result of the findings derived from a “game-theoretic model” of bilingualism. Since the languages were considered as perfect substitutes, the game easily demonstrated cascading phenomenon since the subsequent players tended solely to choose the majority language. The economists found 3 different Nash equilibriums to the game, no learning, or the complete learning of one language by the other language speakers. The various equilibria reached depend on the costs of learning. The writers argued for the subsidization of only majority language learning to minority speakers as the most efficient solution.
This of course discounts a lot of non-economic factors of languages as well as some economic ones. In particular, I would be curious as to the effect of wealth behind a language, perhaps as GDP(s) per speakers, has on the incentivizing of learning it. Intuitively I feel there would be a positive connection, but it would be interesting to try and quantify, especially as non-english speaking nations are gaining much more economic sway globally. I imagine there is also a strong proximity effect in place which will probably be the cause of many SE asian individuals taking up Mandarin over English as a second languages as more money flows out of China southward. Lastly there is the question of whether complete learning or one global language is an efficient equilibrium considering the costs that would go into it and potential intrinsic and extrinsic losses from the disappearance of a mode of communication. In an increasingly global and diversified world it will be very interesting to see how we decided to exchange ideas.