Where the government intervenes in the opposite direction, the transition can be much more rapid. Visiting Strasbourg (in Alsace, France), people's surnames, street and place names, and the local cuisine are all German, but nobody speaks a word of it. It was amazing (and slightly depressing) to see how in 2-3 generations a city could forget the language it spoke for nigh-on 1,500 years.
The way things are going, I wouldn't be surprised if Dutch were considered a dying language 50 years from now.