A fun for giggles future history takes the new U.N.
population projections for 2100 (which have much
more realistic migration assumptions, although they
still assume it halts by 2050, and assume that TFRs
decline and then modestly increase) and combine
them with a more realistic French projection of 108
The numbers are kind of fun.
Further down the thread, james_nicoll pointed out that the sheer number of African countries where French is used and their high rates of population growth also suggests that French is going to remain a significant world language for some time to come.
Both of these projections assume that things will remain equal, granted, and expecting things to remain stable for a century is a fool's calculator-equipped meanderings. Mind, these meanderings do make for an interesting future.