This is interesting because John - the author of Revelatioin - would have written this in his native language, which was Hebrew - not Greek which most translators used.
AVW. Jesus spoke Aramaic and probably Hebrew, which at the time was probably used mostly by Jewish Priests.
John would have spoken Aramaic and Koine Greek. Since he was in the Mediterranian world he used Koine Greek.
He may very well have written it in Greek because that was the Lingua Franca of the Mediteranian world at the time.
Greek was also the language the common people in Judea read the Scriptures in since they spoke Aramaic rather than Hebrew by that time and the Greek Septuagint was the only official translation authorized by the scribes prior to the life of Christ.
If you are interested in the history of the scriptures up to and after the time of Christ I recommend this paper written by an expert as part of a debate: https://www.dropbox.com/s/rxo3o2jh88iemz0/A_Response_to_Gail_Riplinger_re_The_Art_of_Deceit....pdf?dl=0