The Superlinguists: Monolingual societies

Simon Calder meets speakers of indigenous languages (like Welsh in Britain), of dialects (like Moselfrankish in Germany) and vernaculars (like African-American Vernacular English, in the US). These speakers all use the mainstream language every day, but code-switch to their variants, questioning whether their societies are monolingual. Is there even something sinister and oppressive to the idea of monolingualism?