As far as languages go, Theodore was multilingual from the start - as a boy he alternated between the house of his German uncle and that of his French stepfather, Mr. Marneau. Despite being notionally “a German,” he was just as fluent in French as German and probably used French more often. Being bilingual from childhood is probably part of the reason he was so good with languages.
Theodore learned Latin and Greek at the Jesuit school he attended, and when he was a page to the Duchess of Orleans at Versailles he learned Italian from the court tutors. He eventually started working as an agent for the Swedes, and learned Swedish well enough that he could apparently pass for a Swede abroad. He was posted as a spy in London and lived there for some years, becoming fluent in English. Later he had a colonelcy in Spain where he picked up Spanish (assuming he had not already learned it somewhere else along the way). Although I guess I didn’t mention Dutch in the list of languages I posted earlier, that’s an oversight - he certainly knew Dutch. He spent a lot of time in Amsterdam and the Hague, was often on Dutch ships, and had many friends and contacts in Dutch government and society. Given how easily he picked up other tongues it seems almost impossible that with all that time spent in the Netherlands he wouldn’t have learned the language.
He had partial knowledge of a few other languages. During his time as a Swedish agent he was reportedly involved in negotiations between Sweden and Russia and probably knew some Russian, but given that he never set foot in Russia nor had much occasion to use the language later in life I’m not sure how competent he would have been in the 1740s. Theodore spent some time in Lisbon and may have known some Portuguese. His Moroccan friend Hamet appears to have taught him some Arabic for use in reading alchemical texts in that language, and we know that his good friend Ripperda knew Arabic, but whether Theodore knew enough to hold a conversation is unclear. He was also involved with a number of Jewish alchemists and mystics who did Kabbalic study and may have known some Hebrew.
Theodore is probably about as fluent in Italian as you can get without being a native speaker. He served as a spy in Rome, spent some time working at the Medici court in Tuscany, and (ITTL) reigned on Corsica for the better part of a decade. He may have a mild accent, but if he does it’s probably a
French accent given that he learned Italian from a tutor at the French court. The Corsicans have their own dialect of Italian, but it’s easily understood by a standard Italian speaker. The Corsicans might have a few words or phrases that are new to him and his courtly Italian, but otherwise he can easily communicate with of his subjects.
Although Theodore did know Greek, I don’t know whether the sort of Greek taught in a Jesuit school (presumably the Koine Greek of the Bible?) would enable him to hold a conversation with a Greco-Corsican speaking in an 18th century Maniot dialect. Someone with more knowledge of the Greek language would have to tackle that question.
Random, ridiculous thought - would llamas be useful as a beast of burden more suited than the mule for he mountainous regions of Corsica?
I don’t really know anything about llamas, but my instinct is that importing American livestock is probably not the most economical solution to transport problems. The Corsicans already have their own distinct breed of donkey (from which their mules are made) which still exists today. What Corsica really needs is a road network suitable for wagons and carriages, as its roads are almost all just narrow mule-tracks. Naturally a railroad would be helpful as well, but that will have to wait for at least another century.
Also, how do you pronounce Ajaccio in Italian/Corsican?
The name of the city in the Late Middle Ages was
Aiazo, and the Early Modern Period it was
Addiazzo in Italian or
La Hiace in French, although both of those spellings were still in use in the 18th century. The Genoese spelling was
Ajaccio, but it was pronounced and sometimes spelled
Aiaccio, and the Corsicans spelled it
Aiacciu or occasionally
Aghjacciu. ITTL the city’s name will probably stay
Ajaccio or perhaps be changed to
Aiaccio, but either way I suspect it will indeed be pronounced like “ayachio” and definitely not like the modern French pronunciation.