There are currently about 7,000 languages in the world. Imagine that for some reason (alien overlords, global dictatorship, whatever…) humanity has to choose the 50 most important ones. Those selected will not only be preserved, but actively used forever (the mechanisms that would make this possible are irrelevant). All the others will instantly disappear without a trace.
Which criteria should we use to determine the 50 languages that will make the cut ?
Demographics come to mind immediately, but this quickly runs into problems. While lists of the most-spoken languages are readily available, they do not seem to overlap perfectly, due to different methodological approaches. For instance, some mutually intelligible languages, such as Hindi-Urdu and Malay-Bahasa Indonesia, are sometimes counted as distinct, sometimes as one. Opting for the latter would allow a couple more languages to be saved, but would it be politically acceptable ?
An unexpected critieria would be the amount of translations done. According to David Bellos, about 50 languages account for almost all of the translations made worldwide. While I have not been able to find the list, I suspect it would more or less overlap with demographics, but there might be some big surprises. Which brings me to the next criteria.
Some languages hold considerable prestige for literary or religious reasons. It seems that there are almost no native speakers of Sanskrit, but I doubt that the people of the Indian subcontinent would be OK with it vanishing entirely. Should we take this into consideration ?
Besides, some languages are important from a scientific point of view. Depending on the methodology, Malagasy, with its 25 million speakers, falls either just above or just below the 50 languages cutoff point. Should the fact that it is the westernmost Austronesian language, with a unique Bantu substratum earn it a ticket for eternal preservation ? But then, should we consider Pirahã, a language famous for its highly unusual features, despite the fact that it is only spoken by at most 400 people ?
In my opinion, all of these criteria are valid and should be considered when coming up with final list. The real question is, how do we weigh each of them ?