Linguists and the retreaval of lost languages

Many times those languages that died out in order for most of the modern languages to survive. When I watched the movie, I never knew that there would be a language that is spoken is Siberia. There is a misunderstanding that whenever a person speaks a certain language, he's from that country. For example, I'm from Kiev, Ukraine. Whenever people hear me speak, it's mostly English with Russian accent, which I still have. They don't know that I spoke Russian since I lived in Ukraine, since Russian dominated Ukrainian, and it was spread easily. For those people who were forced to give up their native language and speak the conquered language, whenever it might be, English, French, Spanish, Russian, you name it, its very hard to remember it after a long period of time. So, retreaval of these language tells us more about the ancestors of the language and where they came from.