There is indeed a positive correlation between the political and economic muscle of a particular language-speaking country and its popularity. Centuries back when India was Muslim dominance of Persian and Arabic languages was a phenomenon. It goes to the colonising genius of the English that primarily accounts for development of English language as the veritable lingual franca of the world. Even UK's stunted and truncated status as a political and economic entity has not prevented English from being dominant as an international language because one of her former colonies has risen to the dizzy heights of a super power. As for Americans growing interest in mastering Chinese I shall only repeat what the former prez once famously said:"It's all economics,stupid!".
Totally totally true!!! Tomorrow, if Pakistanis start ruling the economics world, Americans would start learning their language very quickly! After all, its economics! LOL