The last lingua franca is English. Nicholas Ostler (who speaks 26 languages) describes how previous lingua francas – Greek, Persian, Latin – were displaced once the need for them vanished, and he predicts the same demise for English as a world language as the pre-eminence of the USA fades. But what will replace it? Nothing, that's why it's the last.
Technology will make translation and interpretation ever easier; lingua francas won't be needed, which may be good news for endangered languages. The Last Lingua Franca is formidably erudite but also very readable. It offers the simple pleasure of listening to an expert discourse on his specialist subject.