Does "cerebellar agenesis" sound English to you? Almost all things in medicine have almost universal names derived from Greek and/or Latin. Of course, until a few hundred years ago it was because those were the languages of science; these days the terms still fill the same purpose as they did back then - providing a common vocabulary for people from diverse origins.
Yes, "cerebellar agenesis" is an English term. Having Latin and Greek etymologies doesn't make the words Latin or Greek; "cerebellar" isn't even a legal Latin adjectival form.
Here are the titles of the wikipedia article "Cerebellum" in some other languages:
Lillehjerne (Danish)
Kleinhirn (German)
Parengephaliδa (Greek - Παρεγκεφαλίδα if you can read Greek)
Cerebelo (Spanish)
Cervelet (French)
Otak kecil (Indonesian)
Smadzenites (Latvian)
Kisagy (Hungarian)
Beyincik (Turkish)
Xiaonao (Chinese - 小脑)
Nobody's copying the English word (well, Tagalog and Malaysian are) -- they're all using their own native terms for "small brain".
Yes, the cerebellum is know as 'lillehjernen' in Danish and that word is the only word most people know for it. Nevertheless, doctors learn the word cerebellum so they can read what doctors in other countries write.
They also use the word amongst themselves. Googling for 'cerebellum ugeskrift for læger' gives plenty of hits. Likewise for 'cerebellar ugeskrift for læger'.
They might write 'agenese' instead of 'agenesis', though.
