Let me guess - you don't live in a region where multiple languages are routinely spoken/heard in daily life? I live in a city of 80,000+ people in the province of Alberta, Canada - not a large city by many peoples' standards, but still the 3rd largest in the province. On any given day I can hear English, Spanish, Mandarin/Cantonese, whatever non-English language is spoken in the Philippines, the dialect of German spoken by the Hutterites, one or two Middle Eastern languages, and there's a large Cree reserve nearby. Oh, and even though Alberta is in the western part of the country, I do hear French on occasion. Canada has a shameful legacy of deliberately trying to stamp out the First Nations languages by forcing the children into residential schools. They were taken away from their families and forced to assimilate into a culture that was completely alien - and cruelly punished for speaking their own languages. Now that the residential schools have been shut down and the government has been attempting (in fits and starts) to make amends for many decades' worth of attempted cultural genocide, some Native groups are making a deliberate effort to relearn their original languages. Time is of the essence, though, since in many cases there are just a few people left alive and able to teach the younger generations. A language is more than just what people speak in everyday situations and what may be written down - when a language goes extinct, so does the knowledge that was only passed along via oral traditions. If something was never written down and the language is lost, the knowledge is lost. Forever. As an anthropologist/historian, I find that tragic. So yes, languages do undergo revival. It takes considerable work and dedication, but it is possible. I can't believe that the Federation (Starfleet in particular) wouldn't hang on to linguistic knowledge, for the sheer pleasure of having that knowledge available, never mind the considerable work done by linguists, anthropologists, historians, diplomats, etc.