My history teacher told me that French used to be the global lingua franca and language of diplomacy. Was this because France was very powerful (Napoleon?), and when did people start using English instead?
French was the lingua franca of diplomacy, especially in the West largely for reasons of colonialism and perceived cultural superiority. Educated people very often learned French.
But, interestingly, that's not the lingua franca that "lingua franca" refers to. "Lingua franca" refers to the language of the Franks. It was a pidgin and, actually, largely not French. It was in use around the Mediterranean for trade and diplomacy from the middle ages until the 19th century.
hi! not discouraging anyone from addressing your questions directly, but FYI you may find some useful info in this section of the FAQ*, specifically in the subsections Other lingua francas (which has one thread on French) and When did English become an international language?
Cross-cultural communication and lingua francas
*see the link on the sidebar or the wiki tab
The era of French economic and cultural power actually predates Napoleon - it comes from the ancien regime, especially the era of Louis XIV, when elites around Europe tended to look to France for high culture.