Agatha H. and the Clockwork Princess reveals that the common language of the empire is actually Romanian. Scientists are often also skilled in Latin, Greek, and German. English is the common language for trade, French is the common language for diplomacy, Arabic is the common language of discussion for some mathematics, and Russian is the common language for jokes. Due to her secretarial duties for Tarsus Beetle, Agatha is fluent in them all.
Possibly relevant outside information Edit
The term is a neologism modelled on lingua franca, which is any language widely used beyond the population of its native speakers. The term comes from the Italian, literally meaning 'Frankish language', originating from the Arabic custom of referring to all Europeans as 'Franks' from the cultural exposure during the Crusades.
The original lingua franca (used in the Mediterranean from the 11th to 19th centuries) was mostly Italian with a broad vocabulary drawn from Persian, French, Greek and Arabic; one may infer from this that the lingua europa likewise has a heavy infusion of Transylvanian-dialect Romanian (from the Baron's homeland and starting point of pacification), langue d'Oc from the south, and French and Alsatian from the west.
In parallel to Europan use, in our world, German (from Wikipedia), "...served as a lingua franca in large portions of Europe for centuries, mainly the Holy Roman Empire.... From about 1200 to 1600, Middle Low German was the language of the Hanseatic League which was present in most Northern European seaports, even London.... During the 19th and 20th centuries, Germany was leading in the sciences — particularly in physics, chemistry and sociology, winning many Nobel Prizes — and the language was also used in international business and politics."