80 years ago the Lingua Franca for diplomacy was French. In fact, French dominated diplomacy from the 17th century until WW2. English didn't start getting used in non-English diplomatic circles until after WW1 (it was quite significant when the Treaty of Versailles was written in both English and French). French has been eclipsed by English, but it is still popular (it is the second most used language in the UN and the EU).
For science and technology, Latin used to dominate. Once people stopped publishing in Latin, three dominant languages appeared: English, French, and German. Which was dominant depended on the field being discussed. Before WW1, German may have been the largest of the three, but after WW1, English was noticeably more dominant (and has only continued to grow).
For business, the general rule is that whenever possible the seller speaks the buyers language. 80 years ago, there were several useful intermediate languages that could be used to facilitate business. The most common would be English, French, and Arabic. I don't know that German was used much outside of Europe and the few German colonies. French was probably the smallest here, since outside of Europe it was most spoken in Africa, where it had to compete with Arabic as a language of trade. There are plenty of other languages which are influential at a regional level, such as Chinese, Russian, Spanish, and Swahili, but these haven't had much of an impact globally. Due to its size and economic might, I expect that Chinese will become more influential in the future, and it will slowly become more significant outside of Asia. I don't see Spanish moving outside of Europe and the Americas, at least not in the short term.