>>17763196 (OP)It's refered to as "Lingua franca".
In Europe is was Latin for most of European history after the fall of Rome, only the 0.01% of the educated people could read and understand latin. A major reason why the reformation succeeded in spreading was because Martin Luther wrote the bible in the local bible, which was practically unheard of.
Lingua franca later abandoned Latin and instead became French, up until the mid-1950s when it became English.
French is still used as the "traditional" language among state diplomats today, even though English has been globally embraced as the Lingua franca.
So whenever you read about for example Nevile Henderson and Ribbentrop communicating in the leadup to ww2, they are speaking in French, not English or German.
In Asia, the Lingua franca was obviously Chinese since China was a polticial, cultural and economical gravitation of east Asia, tho not in the same sense as Latin was to Europe, since the Papacy essentially had rule over the continent (with a few exceptions like Lithuania and Byzantine).