Upon his arrival at the Nepal-based magic school Kamar-Taj, Stephen Strange finds this particularly complicated given the damaged nerves in his hands, but his impressive intelligence eventually allows him to pick up the practice.
It’s a language that’s spoken not so much verbally as it is through specific articulated gestures performed by a sorcerer’s hands.
The information about these spells are kept in books that are stored in the library at Kamar-Taj, and require deep study for usage.
Principally, the use of magic in the Marvel Cinematic Universe is through recitation of an ancient language that, according to the Ancient One, the original users called “spells.”