Lingua francas have been adopted to unite peoples within a country (e.g., Mandarin Chinese in China or Modern Hebrew in Israel), but I'm curious to know more of the political implications when lingua francas expand opportunities for international communication (or their abandonment reduces them).
I've asked for economic implications at https://economics.stackexchange.com/questions/3270/economic-consequences-of-a-lingua-franca which of course can tie into politics, but I'm specifically interested here in asking about social ramifications.
I'd be interested both in positive examples in which a lingua franca is attributed as having improved relations by an increased opportunity for cultural sharing, exchanges, etc., but also any challenges, such as providing the masses with an increased exposure to values which contradict theirs (e.g., being made more acutely aware of another country's differing religious beliefs) and increasing tensions as a result.
Also welcome would be any instances in which a country closed itself from a lingua franca (as for anti-colonial reasons), leading to perhaps poorer relations with countries that formerly shared the common language.
Although I'm most interested in precipitous changes brought by the adoption of a new official language in modern times, I'd also be interested in analyses on the ongoing effects of how a shared language impacts disparate cultures such as Commonwealth of Nations countries.