A huge number of people around the world speak at least a local or family language as well as a "language of wider communication"; depending on their situation, they may easily be fully fluent in both. So a preservation-oriented policy or culture would encourage this kind of bilingualism. This was also the traditional view of the Esperanto movement, which wanted Esperanto to be the unique world-wide language of communication while preserving all of the existing languages through bilingualism. [Edit: A commenter below says this approach is less traditional in Esperantism than I thought.] But a similar result could be achieved with, say, English as the main world language, or with people speaking just a handful of regional languages as well as their family languages.
One challenge to bilingualism is that a lot of governments don't like it (for many different reasons), while another challenge is economic. Bilingualism is very possible, but people in various circumstances could easily stop viewing it as desirable or important, because language A will get them a job in the city and language B won't, and each language takes at least some time and effort to use and keep up with. (Or if people habitually move far away from their families, they'll have fewer opportunities to practice speaking with their families.)
Anyway, there are significant tensions here, but these possibilities don't have to be mutually exclusive as long as people continue to care about their family and local languages and have access to social situations that give them opportunities to use them. And it can even be government policy to try to preserve minority languages while also promoting bilingualism in other more widely-spoken languages.