How did Doctor Strange understand the writings in the Book of Cagliostro? Just by reading the book he learned how to make hand signs to control the power of the Eye of Agamotto.
It seems clear that whatever language the Book (and others) were written in was learned by Stephen Strange during his somewhat lengthy stay at Kamar-taj.
He's already read several other obscure magical books which may or may not have been in English such as
The Book of the Invisible Sun.
Key of Solomon.
When moving on to more "advanced" works Wong says...
WONG: This section is for Masters only. But at my discretion, others may use it. We should start with Maxim's Primer*. How is your Sanskrit?
STRANGE: I'm fluent in Google Translate.
WONG: Read it. Classical Sanskrit.
*A primer (in this sense usually pronounced is a first textbook for teaching of reading, such as an alphabet book or basal reader. The word also is used more broadly to refer to any book that presents the most basic elements of any subject
Please remember that Stephen(Dr. Strange) highlights the fact that he has a very good photographic memory, which helped him a lot in his medical career. (I suppose that he saying this to The Ancient One was actually the story-writers' way of logically stating the fact that Stephen is good in learning new languages.)
It basically concludes that it was easy for him to remember new languages (remembering the alphabets, word formations etc as photos).
Also, no matter how old a book is, certain translations or helping aids are passed down the generations along the book as well, most importantly, the skill to read a language is passed along generations, no matter how much specific group of people may be in it, they can always enable others to do the same.