How can language play a role as eighter unifying or devolutionary force for a country?

Why is a particular language like English came to be distributed the way that it is around the world? Are the processes of globalization influencing the changing distribution and significance of the language? if so why? How can language play a role as eighter unifying or devolutionary force for a country? 

http://www.pbs.org/frontlineworld/rough/2008/08/china_kung_fu_e.html

http://www.pbs.org/frontlineworld/rough/2007/12/philippines_hav.html

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