In what language did St Paul write his epistles

Greek
An epistle (/ᵻˈpɪsəl/; Greek ἐπιστολή, epistolē, "letter") is a writing directed or sent to a person or group of people, usually an elegant and formal didactic letter.

The epistle genre of letter-writing was common in ancient Egypt as part of the scribal-school writing curriculum.

The letters in the New Testament from Apostles to Christians are usually referred to as epistles.

Those traditionally attributed to Paul are known as Pauline epistles and the others as catholic (i.e., "general") epistles.

https://en.wikipedia.org/wiki/Epistle