but really, I think we stand a good chance of some languages dying out because everyone learns English.
In reality, people will need (and correspondingly learn) English less because it won't be necessary as a lingua franca. Eventually all these apps / devices will be able to interpret between a much wider range of language pairs. Linguae franchi will still be necessary for some of the more exotic pairs (and for now that could be something like Greek to Finnish, but eventually all the European languages will be well taken care of).
I also don't think English threatens other languages to the degree that people think. It might leave its trace but it's unlikely to take over all together. People are invested in their own languages in an emotional way so to suggest that English will take over in all walks of life seems unrealistic. You only have to look at the example of 17th-century German. Many people in that era learnt French for reasons of fashion and power, and many French words entered the language, but there was a backlash and eventually German moved on from that. Nowadays English occupies the place of French in that again, people are learning English for reasons of fashion and power. It does leave its mark in terms of Anglicisms and new vocabulary, and certain parts of society are using English copiously (eg. education, business), but I don't think German is endangered per se. Parents still tuck their kids in at night with German lullabies. Politicians still curse each other in German. For me, it's not going anywhere.
Lavandula